Data centre cooling systems

ABSTRACT

A method of cooling a data center having at least one hot aisle ( 145 ) and at least one cold aisle ( 144 ), including the steps of producing cooling air having controlled to have temperature and relative humidity within certain pre-defined limits; supplying the cooling air to a plurality of items of IT equipment ( 143 ) located in the data center between the cold aisle and the hot aisle; measuring the velocity of air flowing from the hot aisle to the cold aisle through an opening ( 150 ) between the hot aisle and the cold aisle; and controlling the rate of supply of cooling air to the items of IT equipment in dependence on the velocity of air so measured.

BACKGROUND OF THE INVENTION

The present invention concerns data centres and a method of coolingequipment in a data centre. The invention also concerns apparatus forcooling a data centre.

A data centre is a late 20^(th) Century development that has grown as aresponse to the increasing demand for computer processing capability anda recognition of the importance of information technology (IT) in theplace of every business and organisation today. Whereas smallerorganisations have sufficient processing power with laptops, PCs andoccasionally servers, larger organisations require higher capacitycentralised processing to serve a wide range of needs and applications.A few years ago this capacity was supplied by large mainframe computers,but more recently the method used has been to provide data centrescomprising many networked computer servers known as “blades” installedin racks enabling controlled and modular expansion of capacity. Theracks also typically house storage systems and/or telecommunicationsequipment such as routers to handle data flow between the computerservers and data flow between the data centre and the outside world.

One key problem faced is how to cool a data centre effectively andefficiently. In a traditional data centre arrangement the racks are laidout in rows. Cooling is provided by direct expansion (DX) or chilledwater cooling plants. The cool air produced by these units is entrainedthrough an underfloor plenum and exits through floor grilles at thefront of the IT rack rows. The IT products installed in the rackscontain integral fans which draw the cooled air from the front acrossthe circuitry and heat is exhausted via vents in the products to therear. In certain arrangements of the prior art, the separation betweenthese IT racks creates a ‘hot aisle’ into which air is expelled by theIT products in the racks and a ‘cold aisle’ from which cooler air isdrawn into and through the IT products by their integral fans.

A typical arrangement of the prior art is shown schematically in FIG. 1of the attached drawings. Thus, the data centre includes a rack room 1defined by walls 2 in which two sets of racks 4 for IT equipment areaccommodated. The IT equipment in the racks 4 generate heat, representedby arrows 6. The cooling of the IT equipment is achieved by introducingcold air, via a floor void, into the room by means of air conditioningunits, the cold air being represented by arrows 8.

In certain data centre arrangements it is important that the volume ofcold air supplied is equal to or greater than that drawn through theservers by their internal fans. If the volume is not sufficient, thenthe servers can draw in warm air from other areas of the data centre,possibly resulting in the IT equipment overheating. In a traditionaldata centre this is generally avoided by supplying significantly morecold air than the servers actually need at any given time. It will beseen that this typical data centre arrangement is not particularlyenergy efficient.

The drive for more efficient use of power has given rise to a need tomake the cooling regimes used in data centres more efficient, as coolingof equipment typically contributes significantly to the power used by adata centre. For example, the power usage in certain data centres mayrequire between 1 kW and 2 kW of power for every 1 kW of power used topower the IT equipment, a significant proportion of which would berelated to cooling.

In recent years, data centre designers have sought to improve energyefficiency with cooling systems that attempt to tailor the amount ofcooling air delivered to the actual requirements of the servers at anygiven time. U.S. Pat. No. 6,283,380 (IBM), for example, describes asystem for automatically controlling the volume and distribution ofcooling air supplied to IT equipment in a data centre based on the dataprovided by a network of temperature sensors deployed at variouspositions around the rack room. The temperature data is input into acomputer simulation of the rack room and the predictions of thissimulation are used to adjust the speed of the fans supplying thecooling air. This system has the disadvantages that it requires asignificant number of both fans and temperature sensors to bedistributed around the data centre in order to achieve effective airflowcontrol, and the simulation uses significant computing power.

An alternative method is to measure the pressure in the “cold aisle”area at the front of the racks and to adjust the amount of cold airsupplied from the CRAC unit so as to maintain a constant pressure. Thepressure at the front of the racks will vary according to whether theamount of cold air being supplied is too much or too little to meet thedemand of the servers. If too much cold air is supplied, the pressurewill increase, whereas if too little is supplied it will decrease.Keeping the pressure constant therefore means that the amount of coldair supplied by the CRAC unit is appropriate to the demand of theservers at a given time.

U.S. Pat. No. 6,694,759 (Hewlett Packard) provides an example of thismethod in which pressure measurements are also used to adjust the ventsthrough which the cold air exits a cold air plenum, providing an extralevel of control over the airflow. As this document explains, however,the pressure within the rack room and within the plenum is highlynon-uniform, and fluctuates unpredictably. As such, a complex network ofsensors and vents is required in order to provide adequate airflowcontrol.

Efforts to improve energy efficiency by separating the flows of hot andcold air in a data centre have also become popular recently. This may bedone by, for example, adding baffles across the top of the hot and/orcold aisles, with doors or further panels across the end of the aisle. Abaffle arrangement is for example proposed in WO 2006/124240 (APC). Indata centres where the hot aisles and/or the cold aisles are enclosed,the pressure differential between the front and rear of the racks can belarger than the situation where the hot and cold aisles are not sealedoff from one another. In such data centres if the pressure in the coldaisle becomes lower than the pressure in the hot aisle, warm air fromthe hot aisle can be drawn back through the servers since all other airpaths are blocked. This situation is likely to lead to the serversoverheating. In data centres with encapsulated hot and/or cold aisles,it is therefore even more important to ensure that the pressure in thecold aisle is always greater than that in the hot aisle. GB2466178(Hewlett Packard) (filed earlier but published later than the earliestclaimed priority date of the present application) describes a method ofmonitoring this pressure differential in a data centre where the coldaisles are supplied with cooling air from an underfloor plenum, whichinvolves measuring the air flow velocity through a small opening betweena hot aisle and a neighbouring cold aisle.

A further way of improving the energy efficiency of a data centre is touse cool air from outside the data centre for cooling the IT equipment(known in the art as free cooling) whenever the ambient conditionsallow, and/or using adiabatic cooling instead of mechanical cooling.WO2010/075358 (Amazon Technologies) (filed earlier but published laterthan the earliest claimed priority date of the present application), forexample, describes a data centre cooling system having both mechanicaland adiabatic cooling apparatus, which can be operated in various modesutilising combinations of adiabatic, mechanical and free coolingdepending. A disadvantage of prior art combined systems such as that ofWO2010/075358 is that although they control what type of coolingapparatus is used depending on ambient air conditions, they are notsophisticated enough to be able to tailor the interaction of the varioustypes of cooling apparatus so as to achieve optimum, or close tooptimum, efficiency.

The present invention seeks to provide an improved method and apparatusfor cooling a data centre. Alternatively or additionally, the presentinvention seeks to provide a system for cooling a data centre thatmitigates one or more of the above mentioned disadvantages.

SUMMARY OF THE INVENTION

The present invention relates to various aspects of a method of coolingIT equipment and/or aspects of a data centre for housing such ITequipment. The invention also relates to a control apparatus forcontrolling the performance of such methods. Embodiments of at leastsome of the aspects of the invention described below relate to datacentres in which IT equipment is, or may be, arranged, for example inracks, between a cold region and a hot region. Cooling air may thus bearranged to pass from the cold region to the hot region over and/orthrough the IT equipment to thereby cool the IT equipment. Some aspectsof the present invention relate to the way in which the cooling of theIT equipment is controlled. There are therefore several aspects of theinvention which are independent from one another, but which share commonfeatures. Some aspects of the invention require the provision of asource of cooling air, for example including one or more fans. Someaspects of the invention require the provision of an adjustably sizedaperture arranged to control the rate of flow of cooling air to the ITequipment. In some cases, there is provided an adiabatic cooler. In someembodiments, there is provided an air flow sensor. In some embodiments,there is provided a control system, for example a control unit, forperforming control processes. In some embodiments, there are providedmeans for determining the psychrometric characteristics, for example therelative humidity and temperature, of air, for example air from outsidethe data centre. Various aspects of the invention will now be describedin further detail.

The present invention provides, according to a first aspect, a method ofcooling a data centre having at least one hot region and at least onecold region, wherein the method comprises the steps of:

producing cooling air, preferably controlled to have temperature andrelative humidity within certain pre-defined limits;

supplying the cooling air to a plurality of items of IT equipmentlocated between the cold region and the hot region;

measuring the rate of air flow from the cold region to the hot regionthrough an opening between the hot region and the cold region, and

controlling the rate of supply of cooling air to the items of ITequipment in dependence on the air flow rate so measured.

The rate of air flow from the cold region to the hot region may bemeasured by measuring the velocity of air flowing through an openingbetween the hot region and the cold region. There may be more than oneopening between the cold region and the hot region. Where there is morethan one opening, the openings need not connect the cold region to thesame hot region. The openings may connect the cold region to two or moredifferent hot regions. Where there is more than one opening, the rate ofair flow from the cold region to the hot region through each opening maybe measured. In such a case, the rate of supply of cooling air may becontrolled in dependence on the average of the air flow rates someasured or in dependence on the highest of the air flow rates someasured, or in dependence on some other calculation using one or moreof the combination of the air flow rates so measured.

The IT equipment may be arranged in racks. The hot region may be in theform of a substantially enclosed region. It may be in the form of aduct. It may be in the form of a space bound by a floor, a ceiling andone or more walls. The floor may be level with the bottom of the racks.The hot region may be in the form of an over-floor air duct. The ceilingmay be level with the top of the racks. The hot region may be a hotaisle, for example a hot aisle that acts as an over-floor air duct. Thecold region may have similar such characteristics as the hot region. Forexample, the cold region may be in the form of a substantially enclosedregion. The cold region may be in the form of a duct. The cold regionmay be in the form of a space bound by a floor, a ceiling and one ormore walls. The cold region may be in the form of an over-floor airduct. The cold region may be a cold aisle, for example a cold aisle thatacts as an over-floor air duct. The step of supplying the cooling air tothe plurality of items of IT equipment may be conducted by means of anover-floor air duct, for example defined in part by the cold region. Theover-floor air duct supplying cooling air may extend all the way fromthe source of the cooling air to the plurality of items of IT equipment.The over-floor air duct may include one or more adjustably sizedapertures arranged to control the rate of flow of cooling air to the ITequipment. The over-floor air duct may be in the form of a corridor, forexample a personnel corridor. Thus, the cooling of the items of ITequipment may be achieved by introducing cooling air, via a route notbeing via a floor void, into the cold region.

The hot region may be a hot aisle, and/or the cold region may be a coldaisle. Advantageously, the step of controlling the rate of supply ofcooling air in dependence on the air flow rate so measured enablesarrangements in which only the amount of cooling air actually needed bythe IT equipment is supplied at any given time. In an embodiment of theinvention, this allows an arrangement in which the energy efficiency ofthe data centre may be improved by the equipment used to produce coolingair always operating at as low a level as possible. Furthermore, thecooling air requirements of each cold region are preferably determinedbased on a single air flow rate parameter (based on, say, measurementsfrom one or two velocity sensors, for example). A complex network ofsensors and simulations may not therefore be needed.

The method may further comprise the steps of producing cooling air at afirst rate and subsequently producing cooling air at a second rate (i.e.different from the first rate). The cooling air may be produced by oneor more fans including at least one variable speed fan. The method mayinclude varying the rate at which cooling air is produced, for examplevarying it according to the cooling requirements of the IT equipment atany given time, which may be achieved by means of the measured rate ofair flow through the opening.

The method may further comprise the step of passing cooling air throughan aperture of a first effective size, adjusting the size of theaperture to a second effective size (i.e. different from the firsteffective size), and subsequently passing cooling air through theaperture of the second effective size. The aperture may be an adjustablysized aperture. It may include, or be defined by, a damper. The aperturemay include several damper blades, in which case the area definedbetween a pair of blades or between a blade and the edge of the aperturemay be adjustable. In such a case, the aperture may be considered asextending to cover the space defined by the blades and the area betweenadjacent blades. The effective size of the aperture may be adjusted byrotating the damper blade(s) along their axes. The aperture may belocated upstream of the IT equipment and downstream of the apparatusused to produce the cooling air. Advantageously, the provision of anadjustably sized aperture allows a further level of control over theamount of cooling air supplied to the IT equipment.

The step of controlling the rate of supply of cooling air may includevarying the size of the aperture in dependence on the air flow ratethrough the opening. The step of controlling the rate of supply ofcooling air may also include varying the rate of production of coolingair in dependence on the size of the aperture and on the air flow ratethrough the opening.

The method may include the step of controlling the rate of supply ofcooling air to the IT equipment in dependence on at least one criterionconcerning the air flow rate through the opening. Preferably thecriterion is defined such that the air pressure in the cold region willalways be slightly higher than the air pressure in the hot region.Preferably the criterion is defined such that the air pressure in thecold region will never be significantly higher than necessary togenerate the cooling air flows necessary to meet the cooling demand ofthe IT equipment. The criterion may be defined such that the airpressure in the cold region will never be considerably higher than theair pressure in the hot region. Maintaining at least some air flow fromthe cold region to the hot region through the opening indicates that thecold region is at an appropriate pressure to meet the cooling demand ofthe IT equipment.

In the case where an adjustable aperture is provided, the method may beperformed such that if the air flow rate through the opening is below afirst pre-set level, the size of the aperture is increased. Thisincreases the amount of cooling air supplied to the IT equipmentdownstream of the aperture. The size of the aperture at any given timeis preferably such that the rate at which cooling air is produced isrelatively low (preferably as low as possible) while still beingsufficient to meet the demand of the IT equipment at that time. Themethod may be performed such that if the air flow rate through theopening is below the first pre-set level and the size of the aperturehas been increased to its operational maximum size, the rate at whichcooling air is produced is increased.

The method may be performed such that if the air flow rate through theopening is above a second pre-set level, the rate at which cooling airis produced is reduced. This may be done by decreasing the speed of thevariable speed fan, which reduces the energy usage of the data centre.The method may be performed such that if the air flow rate through theopening is above the second pre-set level, the size of the aperture isreduced. The size of the aperture may be so reduced in the case wherethe rate at which cooling air is produced has already been reduced to arelatively low level, for example to a level deemed to be theoperational minimum. The first and second pre-set levels may bedifferent. In the embodiment described below the first and secondpre-set levels are equal. The first and second pre-set levels need notrepresent large air flow rates. The first and second pre-set levels maybe low air flow rates, for example an air flow rate where the speed ofthe air flowing is less than 1 ms¹.

The IT equipment may be arranged so that substantially all of thecooling air passes through the IT equipment along air flow pathsinternal to the items of IT equipment. Ensuring that cooling air onlypasses through the inside of each item of IT equipment and does not passin between the items of IT equipment may improve the cooling effect ofthe air on the IT equipment. Blanking panels may be used to prevent airflowing between individual items of IT equipment. Blanking panels may beused to prevent air flowing between the IT equipment and the floor ofthe cold region. Blanking panels may be used to prevent air flowingbetween the IT equipment and the ceiling of the cold region. Blankingpanels may be used to prevent air flowing between the IT equipment andthe walls of the cold region.

The data centre may have a plurality of hot regions and cold regions.Each of the plurality of cold regions may be fed with cooling air via adedicated variably sized aperture. This allows a different amount ofcooling air to be supplied to each cold region, which is advantageoussince the demand of the IT equipment cooled with air from each coldregion may be different. In this case the method may comprise the stepsof measuring the air flow rate through a first opening between a firsthot region and a first cold region and measuring the air flow ratethrough a second opening between a second hot region and a second coldregion, and supplying cooling air to IT equipment, on the one hand,between the first hot region and the first cold region and, on the otherhand, between the second hot region and the second cold region, independence on the first and second measured air flow rates. It will beappreciated of course that there may be more than one opening betweeneach pair of hot and cold regions, such that the measured air flow ratethrough the first opening is combined with, or otherwise used inconjunction with, a measured air flow rate through one or more furtheropenings between the first cold region and the first hot region.

The method may be performed such that if the air flow rate through thefirst opening or the second opening is below the first pre-set level,the size of the aperture associated with the cold region connected tothat opening is increased. The method may be performed such that if theair flow rate through the first opening or the second opening is belowthe first pre-set level and the size of the aperture associated with thecold region connected to that opening has been increased to itsoperational maximum size, the rate at which cooling air is produced isincreased.

The method may be performed such that if the air flow rate through oneof the first opening and the second opening is above the second pre-setlevel and the size of the aperture associated with the cold regionconnected to the other of the first opening and the second opening isnot at its operational maximum size, the rate at which cooling air isproduced is decreased. The method may be performed such that if the airflow rate through one of the first opening and the second opening isabove the second pre-set level and the size of the aperture associatedwith the cold region connected to the other of the first opening and thesecond opening is at its operational maximum size, the size of theaperture in the cold region connected to the opening through which theair flow rate is above the second pre-set level is reduced. The methodmay be performed such that if the air flow rate through the firstopening is above the second pre-set level and the air flow rate throughthe second opening is above the second pre-set level, the rate at whichcooling air is produced is decreased. Thus the amount of cooling airsupplied to each cold region may be tailored to meet, with greataccuracy, the cooling demand of the IT equipment drawing air from thatcold region at any given time. The speed of the variable speed fan maytherefore be able to be reduced to be as low as possible while stillbeing sufficient to meet the cooling demand of the region with thehighest cooling requirements. In such a case, the aperture (orapertures) associated with the region having the highest coolingrequirement id advantageously controlled to be at its operationalmaximum size.

The method may be performed such that it includes the step of defining amaximum rate of production of cooling air in dependence on one or morecharacteristics of the IT equipment, and cooling air is produced at arate less than or equal to this maximum rate. The maximum rate ofproduction of cooling air may be defined in dependence on the powerusage of the IT equipment. It may be defined in dependence on thetheoretical maximum requirement of the IT equipment for cooling air. Itmay be greater than 120% of the maximum requirement of the IT equipmentfor cooling air. It may be greater than twice the maximum requirement ofthe IT equipment for cooling air. Providing such a maximum limit for therate of production of cooling air improves the energy efficiency of adata centre in which the method is performed. This is because insituations where the rate of production of cooling air otherwise mighttemporarily increase by a significant amount unnecessarily, for exampleif the removal of baffles in the cold aisle during maintenance allowedsome cooling air to pass directly from the cold aisle to the hot aislewithout passing through the IT equipment, the provision of a maximumrate will prevent the apparatus producing the cooling air from operatingat too high a level and using an excessive amount of energy.

The cooling air may be produced from supply air, the supply aircomprising ambient air from outside the data centre, or air exhausted bythe IT equipment in the data centre, or a mixture of air from outsidethe data centre and exhaust air. How much, if any, ambient air and howmuch, if any, exhaust air the supply air comprises may be controlled independence on the temperature and relative humidity of the outside air.

Preferably, production of cooling air is controlled to have temperatureand relative humidity within certain pre-defined limits. The control ofthe cooling air to have temperature and relative humidity within certainpre-defined limits may consist of ensuring that the air has the abilityto cool the IT equipment. The cooling air may be produced by adjustingthe temperature and relative humidity of the supply air to be within thepre-defined limits by any one of mechanical cooling, adiabatic cooling,or a combination of mechanical and adiabatic cooling. How much, if any,mechanical cooling, and how much, if any, adiabatic cooling is used toadjust the temperature and relative humidity of the supply air to withinthe pre-defined limits may be controlled in dependence on thetemperature and relative humidity of the outside air. Preferably nomechanical cooling is used for the majority of the time, which meansthat the energy usage of the data centre is very low.

According to a second aspect of the invention there is also provided amethod of cooling a data centre with cooling air, wherein the methodcomprises the following steps:

(a) defining criteria for the temperature and relative humidity of thecooling air, wherein the criteria are a range of temperatures and arange of humidities;

(b) determining the temperature and relative humidity of ambient airfrom outside the data centre;

(c) determining a set point for each of the temperature and relativehumidity of cooling air, the set point satisfying the criteria definedin step (a) and being chosen in dependence on the temperature andrelative humidity of the ambient air;

(d) producing cooling air having temperature and relative humiditysubstantially equal to those set points; and

(e) delivering the cooling air to a region in the data centre to becooled.

The range of humidities at which the cooling air is deemed to besubstantially equal to the set point for relative humidity may be widerthan the range of temperatures at which the cooling air is deemed to besubstantially equal to the set point for temperature. For example insome embodiments a ±2% tolerance on temperature may be too great,whereas a ±2% tolerance on relative humidity would be acceptable. Incertain embodiments, a ±5% tolerance (or even a ±10% tolerance) onrelative humidity may be acceptable.

The method may further comprise the following steps:

(f) removing exhaust air from the region of the building to be cooled,the exhaust air including air heated as a result of heat exchangebetween the cooling air and the region in the building to be cooled; and

(g) choosing how much, if any, of the exhaust air and how much, if any,of the ambient air are used during step (d) to produce the cooling airin dependence on the temperature and relative humidity of the ambientair.

The criteria defined in step (a) may be such that they may berepresented on a psychrometric chart by a single point. The criteriadefined in step (a) may be such that they may be represented on apsychrometric chart by a single line. This line is preferably a line offinite length; however it may be a closed loop. Conditions correspondingto any of the points on the line will satisfy the criteria defined instep (a); however producing cooling air having characteristicsrepresented by some points on the line will require more energy thanproducing cooling air having characteristics represented by certainother points on the line.

Preferably the step of choosing a set point, for example on such a line,for the temperature and relative humidity of cooling air in dependenceon the temperature and relative humidity of the ambient air at a giventime is carried out such that producing cooling air with temperature andrelative humidity at the chosen set point requires no more coolingenergy than producing cooling air with temperature and relative humidityrepresented by any of the other points on the line. This means that themost efficient possible process for producing cooling air is alwaysused.

Step (d) of the method may include a first stage of producing coolingair having relative humidity higher than the set point for the relativehumidity of cooling air and temperature lower than the set point for thetemperature of cooling air, and a second stage of raising thetemperature and lowering the relative humidity of the cooling air soproduced, by passing it through a fan, so that the temperature andrelative humidity of the air are substantially equal to the set pointsfor the temperature and relative humidity of cooling air. The method maytherefore include compensating for the temperature rise and humiditydrop produced when the air is passed through the fan (or fans). It willbe appreciated that the relative humidity may need to be only slightlyhigher than the set point for the relative humidity of cooling air andthat the temperature may need to be set only slightly lower than the setpoint for the temperature of cooling air, in order to reach the desiredset points because the temperature rise (and consequential relativehumidity drop) caused by passing the cooling air through the fan(s) willtypically be relatively low. The temperature rise caused by passing thecooling air through the fan(s) may be of the order of one or two degreesCentigrade. According to a third aspect of the invention there is alsoprovided apparatus for supplying cooling air to a plurality of items ofIT equipment in a data centre having at least one hot region and atleast one cold region. The apparatus may comprise

an opening between the hot region and the cold region;

an air flow sensor located in said opening for measuring the rate atwhich air passes through the opening;

a source of cooling air, the cooling air having temperature and relativehumidity within certain pre-defined limits, and

an adjustably sized aperture located upstream of the IT equipment anddownstream of the source of cooling air.

The apparatus may be arranged such that the rate at which the coolingair is supplied to the IT equipment depends on the air flow ratemeasured by the sensor. For example, the effective size of theadjustably sized aperture may be adjusted in dependence on an output ofthe air flow sensor. The opening may be a one-way duct. For example, theduct may define the opening. Other physical structure, separate from theIT equipment, may define the opening. The end of the duct that joins thehot aisle may be covered by a hinged flap, or the duct may include someother form of non-return valve. This prevents hot air from flowing backinto the cold aisle, and means that a simple non-directional velocitysensor may be used. Alternatively the duct may be a two-way duct, inwhich case a directional air flow sensor may be preferred. The air flowsensor may be in the form of an air velocity sensor. The air flow sensormay be arranged to provide an output in dependence on the volume of airthat passes through the opening per unit time. The air flow sensor maybe arranged to provide an output in dependence on the speed at which airpasses through the opening. Of course, the measure of volume of airpassing through the opening per unit time may be directly proportionalto the speed at which air passes through the opening. The air flowsensor may be in the form of a differential pressure sensor. The openingmay be in the form of a simple orifice, across which a differentialpressure is measured.

According to a fourth aspect of the invention there is also provided acontrol system for controlling the cooling of IT equipment in a datacentre. The control system may include

an input for receiving information about the temperature and relativehumidity of ambient air outside the data centre;

memory for storing pre-loaded criteria for the relative humidity andtemperature of the cooling air, said criteria covering a range oftemperatures and a range of humidities, and

a processor for:

(a) determining a set point for each of the temperature and relativehumidity of the cooling air in view of the pre-loaded criteria and theinformation received about the temperature and relative humidity of theambient air;

(b) determining how much, if any, of the exhaust air and how much, ifany, of the ambient air are used by the source of cooling air to producecooling air having temperature and relative humidity substantially equalto the set points, in view of the information received about thetemperature and relative humidity of the ambient air; and

(c) determining how much, if any, mechanical cooling and how much, ifany, adiabatic cooling is used by the source of cooling air to producecooling air having temperature and relative humidity substantially equalto the set points.

The processor may perform operations (a), (b) and (c) in view of theinformation received about the temperature and relative humidity of theambient air.

Operations (a), (b) and (c) may be carried out by software installedonto the memory of the control system. The control system may beprogrammed with different pre-loaded criteria according to the type ofIT equipment in the data centre at any given time. The control systemmay be arranged for connection to, and control of, a source of coolingair. The control system may be arranged for connection to, and controlof, physical means for varying the amounts of exhaust air and ambientair used in generating the cooling air. For example, the control systemmay be arranged for connection to, and control of, one or more dampers,valves or the like.

The present invention also provides, according to a fifth aspect,apparatus for cooling air, preferably for adiabatically cooling air. Inthe context of the present invention a cooling process may be consideredas an “adiabatic” process if there is no change, or very little change,in enthalpy. It will be appreciated that a cooling process in whichthere is some, for example negligible, heat exchange with the externalenvironment may still be considered, in a practical sense, as anadiabatic cooling process. This apparatus may comprise a cooler havingat least a first section and a second section, wherein the first andsecond sections are individually selectively operable. Air may forexample be cooled by means of passing over or through the one or moresections of the cooler. Each section may itself have a limited, finite,number of settings (for example operating states) for varying the amountof cooling provided by an individual section. The number of settings maybe fewer than five. The number of settings may be two, in that eachsection may itself only be controlled by selectively operating thesection or not operating the section (i.e. simple “on/off” control). Thecooler may have three or more individually operable sections. Preferablythe cooler has ten or fewer such sections. Preferably all sections ofthe one cooler are arranged in parallel (for example, not arranged inseries). For example, an array of such sections may be provided in asingle plane. Sections of the cooler may be arranged adjacent to eachother. The cooler may be in the form of an evaporative coolingapparatus. The cooler may be in the form of adiabatic cooler. Theapparatus for cooling air may comprise an adjustably sized aperture, forexample a bypass damper (i.e. a damper which allows air to bypass thesections of the cooler). Preferably the adjustably sized aperture isarranged in parallel with the cooler. Preferably, the adjustably sizedaperture and the cooler are accommodated in the same airflow channel.Preferably, the apparatus is so arranged that air flowing along thechannel from a position upstream of the apparatus to a positiondownstream of the apparatus must pass via either one or more sections ofthe cooler or the adjustable aperture. The apparatus may be arrangedsuch that in use air is permitted to pass through a non-operatingsection of the cooler. Operation of a section of the cooler may compriseintroducing moisture, for example a flow of water, into the section. Theapparatus is preferably so arranged that the amount of cooling providedby the apparatus depends on both how many sections of the cooler are inoperation and the size of the adjustably sized aperture. Advantageously,this arrangement allows for a much finer degree of control over theamount of cooling (for example the amount of adiabatic or evaporativecooling) provided by the apparatus than could be achieved without usingan adjustably sized aperture. It will be appreciated that this advantageis more pronounced the lower the number of sections and the lower thenumber of operating states (or cooling settings) that each section has.Having such a fine degree of control available may allow the amount ofcooling provided to be closely matched to the cooling demand at anygiven time. Having such a fine degree of control available may allow theoperation of a control regime in which set points for the temperatureand relative humidity of cooling air can be reached and maintained witha high degree of accuracy.

For example, a method of cooling, for example IT equipment in a datacentre, may utilise the aforesaid apparatus by operating at least onebut not all of the sections, and then increasing the amount of cooling(for example providing the same rate of flow of air but at a lowertemperature) by means of reducing the size of the adjustable aperture,thereby forcing more air through the sections of the cooler. The methodmay include a step (as cooling demand increases still further) of thenclosing the aperture completely such that substantially all air passesthrough the sections of the cooler. The method may include a step ofthen causing a further single section of the cooler to becomeoperational.

The method may include a step of performing the cooling method for a settime (thereby introducing a delay) with the adjustable aperture at apre-set position (for example, fully closed) before changing the numberof sections of the cooler which are operational. Introducing such adelay can be useful in maintaining a smooth and/or efficient coolingregime. At substantially the same time (just before, just after, or atexactly the same time) as a new section of the cooler is madeoperational, the adjustable aperture may start moving to a pre-setposition. For example, the adjustable aperture may move to be a position(i.e. size of aperture) at which the amount of cooling provided by theapparatus, once the extra section is fully operational, is substantiallythe same as the amount of cooling provided by the apparatus immediatelybefore both the extra section was made operational and the position ofthe aperture was so moved. In this manner, the amount of coolingprovided by the apparatus can, in effect, be smoothly varied as extracooling sections are made operational. The preset position to which theaperture moves may depend on the number of sections which areoperational.

It will be appreciated that order of the steps of the method justdescribed (in the context of the cooling demand being greater than thecooling provided—i.e. a condition where more cooling is needed) isimportant but that not all steps need be performed. For example, theaperture need not be closed completely before causing an extra coolingsection to become operational.

A method of cooling may similarly include steps which are performed whenthe cooling demand is less than the cooling provided (i.e. a conditionwhere less cooling is needed). For example, a method of cooling mayutilise the aforesaid apparatus by operating at least two of thesections, and then decreasing the amount of cooling by means ofincreasing the size of the adjustable aperture, thereby passing less airthrough the sections of the cooler and more through the aperture. Themethod may include a step (as cooling demand decreases still further) ofthen opening the aperture completely. The method may include a step ofthen causing a further single section of the cooler to becomenon-operational.

The method may include a step of performing the cooling method for a settime (thereby introducing a delay) with the adjustable aperture at apre-set position (for example, fully open) before changing the number ofsections of the cooler which are operational.

At substantially the same time (just before, just after, or at exactlythe same time) as an operational section of the cooler is madenon-operational, the adjustable aperture may start moving to a pre-setposition.

The preset position to which the aperture moves may depend on (a)whether cooling demand is not being met or cooling demand is being morethan met and (b) the number of sections which are operational.

A sixth aspect of the invention provides apparatus for controlling therate of flow of air into, from, or within a data centre (for example tocontrol exhaust of air from a data centre), the apparatus comprising atleast a first adjustably sized aperture and a second adjustably sizedaperture, wherein each adjustably sized aperture is arranged to beindividually controlled. Advantageously, this arrangement allows abetter degree of control over flow rates, for example the rate at whichair is exhausted from the data centre, than can be achieved with anarrangement in which all of the adjustably sized apertures arecontrolled together (so that each aperture is opened or closed by thesame amount in parallel, in contrast to the present aspect of theinvention where the apertures may be opened and closed in sequence).Each adjustably sized aperture may comprise a set of damper blades. Eachadjustably sized aperture may be located at or near the point of exhaustfrom the data centre. The adjustably sized apertures are preferablyarranged in parallel with each other (as opposed to arranging aperturesin series in which arrangement air would pass thorough each of theapertures in turn).

A method of controlling the rate of air exiting a data centre accordingto this sixth aspect is also provided. Such a method may comprise thesteps of providing a data centre having one or more fans generating anair flow and a plurality of adjustably sized apertures arranged tocontrol the rate of air flow, for example from inside the data centre tooutside the data centre. There may be steps of causing air to exhaustfrom the data centre at a first rate during which one or more of theadjustably sized apertures are at least partially open, and then movingone but not all of the adjustably sized apertures to cause air toexhaust from the data centre at a second, different, rate. The methodmay include having one or more apertures in a fully closed position andone aperture partially open, then fully opening that aperture beforestarting to open one of the fully closed apertures. The method mayinclude having one or more apertures in a fully open position and oneaperture partially open, then fully closing that aperture beforestarting to close one of the fully open apertures.

The method may include moving the adjustably sized apertures in sequenceas the flow rate (for example exhaust rate) is varied from a lower rateto a higher rate such that the adjustment in flow rate is effected atlower flow rates by moving a first set of one or more apertures whereasthe adjustment in flow rate is effected at higher flow rates by moving asecond different set of one or more apertures. The method may includevarying the sequence. For example, the method may include using at leastone adjustably sized aperture to enable variation of the air flow rateacross a range of lower rates at one instant and then at another instantusing that same adjustably sized aperture to enable variation of the airflow rate across a higher range of rates not including the lower range.The method may alternatively or additionally be used to control the rateof air entering a data centre. The method may be used to control therate of air moving from one part of a data centre to another part ofthat data centre.

A seventh aspect of the present invention provides a method of accessinga first space in a data centre, the first space having a high airflowdue to the operation of at least one variable speed fan, from a secondspace, the second space having a low or zero airflow. There may forexample be a door between the first space and the second space. Theremay be a relatively large differential pressure across the door as aresult of the airflows in the first space making it difficult and/ordangerous to open the door manually.

The method according to this seventh aspect of the present invention mayinclude the steps of:

operating the at least one variable speed fan at a first (for examplerelatively high) speed;

operating the at least one variable speed fan at a second,pre-determined (for example relatively low), speed for a firstpre-determined time period; and

operating the at least one variable speed fan at the first speed for asecond pre-determined time period after the first time period haselapsed.

The door may be opened during the period when the at least one variablespeed fan is operating at the second speed. This method may thus alloweasy and safe access to areas housing the air treatment apparatus for adata centre while the apparatus is in operation. The second speed ispreferably sufficiently low to enable the door to be opened and closedsafely. The method is preferably controlled by a control unit. Themethod may be used to enable access to the first space by a person toengage in maintenance work. The method may include operating the atleast one variable speed fan at the second speed for a firstpre-determined time period in response to a manually made request, forexample to open the door. For example the request may be provided bypressing a door-release button, lever, or the like.

The present invention also provides, according to an eighth aspect, amethod of operating a data centre in a first mode in which substantiallyall of the cooling done is mechanical cooling, and subsequentlyoperating the data centre in a second mode in which a significantproportion of the cooling done is adiabatic cooling. The method mayinclude a step of providing adiabatic cooling apparatus in an offlinestate and then operating the adiabatic cooling apparatus simultaneouslywith the mechanical cooling apparatus for a pre-set time period. Theadiabatic cooling apparatus may be operated at a level dependent on thetemperature and relative humidity of ambient air outside the datacentre. The method may be performed such that the adiabatic coolingapparatus is initially operated at a level which represents asignificant proportion of the operational maximum of the apparatus (forexample at least 40%, and preferably at least 50%). The method mayinclude a step of reducing the level of operation of the mechanicalcooling apparatus to a level dependent on the temperature and relativehumidity of the ambient air, when the pre-set time period has elapsed.When the adiabatic cooling apparatus is in an offline state for anysignificant length of time, the moisture retaining parts of theapparatus have a tendency to dry out. When initiated there can thereforebe a significant time-lag before the adiabatic cooling apparatus reachesits steady state cooling levels, at any given level of desired coolingcapacity.

Advantageously, this method ensures that the adiabatic cooling apparatusis sufficiently wet to be able to provide the level of cooling required(i.e. to convert ambient air into cooling air of the requiredcharacteristics) immediately after the mechanical cooling apparatus isturned off, despite the time it might take for the adiabatic coolingapparatus to reach its steady state cooling rate when it is broughtonline. The method may therefore, for a short-time, cause the coolingair to be cooler than required as the adiabatic cooling apparatus isoperated in parallel with the mechanical cooling apparatus duringpre-set time period. The pre-set time period may be considered as atransition period between the first and second modes of operation. Thefirst mode of operation (in which substantially all of the cooling doneis mechanical cooling) may be such that the data centre re-circulatesfully the exhaust air and draws little or no ambient air duringoperation. Such a (fully re-circulating) mode may be engaged when theoutside air is very warm or when a fire alarm or VESDA alarm istriggered. The second mode of operation (in which a significantproportion of the cooling done is adiabatic cooling) may be such thatthe cooling air is composed substantially entirely of ambient air fromoutside the data centre (a “fresh air” mode). This method may be usedwhen the data centre is changed from a fully re-circulating mode to afresh air mode because the fire alarm or VESDA alarm has been triggeredwhen there is, in fact, not a fire.

The adiabatic cooling mentioned above may be in the form of evaporativecooling. The eighth aspect of the invention also has application in thecase where heat is exchanged, for example extra cooling is performed,alongside or in combination with the adiabatic cooling. For example, anevaporative cooler may reduce the temperature of the air in a mannerthat is effectively a combination of perfect adiabatic cooling and heatexchange, whether positive or negative. The second mode of cooling maytherefore be one in which a significant proportion of the cooling doneis evaporative cooling, whether or not adiabatic.

According to a ninth aspect, the present invention provides a method ofoperating a data centre in a first mode in which most or all of thecooling is not provided by mechanical cooling and subsequently operatingthe data centre in a second mode in which substantially all of thecooling is provided by mechanical cooling, the method including thefollowing steps:

defining a target value for cooling air produced by the mechanicalcooling apparatus, wherein this target value is significantly lower thanthe temperature of cooling air produced when the data centre isoperating in the first mode;

increasing the target value by a pre-set amount over a pre-set period oftime; and

operating the mechanical cooling apparatus at a level dependent on thetarget value.

The target value may be gradually increased over time to a finalsteady-state target value. By initially setting the target value to avalue that is significantly lower than the final steady-state targetvalue, the mechanical cooling system may effectively be run at a levelsignificantly higher than the level that would be necessary to achievethe final steady-state target value in the most energy efficient mannerpossible. However, by over-driving the mechanical cooling systeminitially, there is less of a risk of an unintentional increase intemperature as a result of lag in the system. The mechanical coolingsystem may be run at a relatively high level temporarily in order toachieve the final steady-state target value in a timely manner.

This method has the advantage that it may act to force the mechanicalcooling apparatus to operate at its maximum level as soon as the datacentre is switched into a mode in which all of the cooling demand mustbe met by the mechanical cooling apparatus. There is therefore a reducedrisk of the mechanical cooling apparatus not being able to meet thiscooling demand because the DX system is operating at too low a level. Ifthe mechanical cooling apparatus were allowed to adjust to the change inmode automatically, the delay in bringing additional DX cooling coilsonline could mean that the temperature of the cooling air supplied tothe servers would rise to unacceptable levels for a period of time.

The second mode of operation in this aspect of the invention (in whichsubstantially all of the cooling done is mechanical cooling) may be suchthat the data centre re-circulates fully the exhaust air and drawslittle or no ambient air during operation. The first mode of operationin this aspect of the invention may be such that a significantproportion of the cooling is not provided by mechanical cooling. It maybe that none of the cooling is provided by mechanical cooling during thefirst mode.

According to a tenth aspect, the present invention provides a method ofoperating a data centre having a smoke detection system and mechanicalcooling apparatus, wherein the smoke detection system has detected smokeand a significant proportion of the mechanical cooling apparatus hasbecome inoperable. The method may include a step of operating the datacentre in a mode in which the cooling air is composed substantiallyentirely of ambient air from outside the data centre and in whichsubstantially none of the cooling is provided by mechanical cooling (forexample, a “fresh air cooling” mode). Whilst on detection of smoke itmay be preferable to operate in a mode in which mechanical cooling isused and in which exhaust air is re-circulated, if sufficient mechanicalcooling is unavailable and there is no fire alarm activated it may bepreferable to utilise a fresh air cooling mode instead. (It may bebetter, in the case of a combination of a mechanical cooling fault andsmoke alarm situation, to keep the IT equipment cool if the risk of fireis still judged as low, than to over-react to a fire-risk and prejudicethe proper function of the IT equipment.) Apparatus suitable fordetecting a fire within the data centre may be provided, for example inthe form of a fire alarm system. In the case where a fire issubsequently detected within the data centre, the method preferablyincludes a step of operating the data centre in a mode in which thecooling air is composed substantially entirely of exhaust air and inwhich substantially all of the cooling is provided by mechanical cooling(for example, a fully re-circulating mode).

Advantageously, this method ensures that the electronic equipment in thedata centre is kept cool enough in the event that smoke is detected inthe data centre and the mechanical cooling apparatus experiences apartial failure. If, as would be the usual response to detecting smoke,the data centre was switched into full re-circulation mode, themechanical cooling apparatus that was still operational would not beable to meet the demand for cooling air and the electronic equipmentcould overheat. However, if an actual fire is detected within the datacentre, it is more important to control the fire risk than to keep theservers cool so the data centre is switched into full-recirculationmode, notwithstanding the reduced mechanical cooling capacity, so thatfire suppressant gas can be deployed.

A final aspect of the present invention provides a method of monitoringthe status of a network of controllers suitable for controllingcomponents of a data centre cooling system. The method may include thesteps of:

providing a primary controller arranged to be able to send signals to atleast one secondary controller and to receive signals from saidsecondary controller;

providing at least one secondary controller arranged to be able toreceive signals from the primary controller and to send signals to theprimary controller;

the primary controller sending a first signal to the secondarycontroller; and

the secondary controller sending a second signal to the primarycontroller.

The method may be performed such that in the case that either (a) theprimary controller does not receive a second signal from the secondarycontroller during a first pre-set time period or (b) the secondarycontroller does not receive a first signal from the primary controllerduring a second pre-set time period (which need not necessarily bedifferent from the first pre-set time period), at least one component ofthe cooling system is instructed by one of the controllers to enter afailsafe mode.

The method may be performed such that in the case that the primarycontroller does not receive a second signal from the secondarycontroller during the first pre-set time period, the primary controllerinstructs at least a first component of the cooling system to enter afailsafe mode. The method may be performed such that in the case thatthe secondary controller does not receive a first signal from theprimary controller during a second pre-set time period, the secondarycontroller instructs at least a second component (which may or may notbe different from the first component) of the cooling system to enter afailsafe mode.

The method may be performed such that the primary controller sends thefirst signal to the secondary controller in response to the secondsignal received from the secondary controller. It will be appreciatedtherefore that the second signal may be sent before the first signal issent. The primary controller is preferably arranged to receive signalsfrom a plurality of secondary controllers.

The fail safe mode may include operating one or more fans at or neartheir operational maximum. The fail safe mode may include moving one ormore adjustable apertures to their fully open state. In an embodiment ofthe invention, the secondary controllers are each associated with a coldregion. In that embodiment, adjustable apertures are associated witheach cold region. The primary and secondary controllers are preferablylinked to each other via a computer network arrangement, such as anEthernet network. Checking the health of the network in the mannersuggested above may be of particular benefit in that a furthersupervisory control panel or interface need not be required for thepurpose of checking the health of the network or for the purpose ofreacting to a detected fault.

The method of the invention, according to any of the above aspects, maybe implemented by a suitably arranged control apparatus. For example,the control apparatus may be arranged to receive inputs, from whichmeasures of outside air temperature and relative humidity may beascertained. The control apparatus may be arranged to receive at leastone input from an air flow sensor located in an opening, for example ina rack of IT equipment, between a hot region and a cold region. It willbe appreciated that other aspects of the invention may require a controlunit, or multiple control units, requiring further inputs. The controlapparatus may control fan speed. The control apparatus may control theadjustment of adjustable apertures. The control apparatus may controlthe use of the cooling apparatus mentioned above (for example theapparatus for adiabatic cooling and/or evaporative cooling). The controlapparatus may control the use of mechanical cooling. It will beappreciated that other aspects of the invention may require a controlunit, or multiple control units, controlling other parts of the datacentre.

The control apparatus may comprise one or more programmable controlunits. The method of the invention may be implemented by means ofsuitably programmed control units. The invention may therefore extend tothe programming of a control apparatus, and for example to programmingof one or more control units to perform the methods of the invention.The present invention thus further provides a computer programcomprising computer program code means adapted to perform any of themethods of the invention, according to any of the aspects describedherein, when said program is run on a programmable controller.

It will of course be appreciated that features described in relation toone aspect of the present invention may be incorporated into otheraspects of the present invention. For example, the method of theinvention may incorporate any of the features described with referenceto the apparatus of the invention and vice versa.

DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described by way ofexample only with reference to the accompanying schematic drawings ofwhich:

FIG. 1 shows a prior art rack room;

FIG. 2 shows a data centre building according to a first embodiment ofthe invention;

FIG. 3 is a partial plan view of a data centre building according to asecond embodiment of the present invention;

FIG. 4 shows a rack room door with variable air flow intake according toa second embodiment of the invention;

FIG. 5 shows a first example psychrometric chart including a controlline and zones;

FIG. 6 shows a second example psychrometric chart including a controlline and zones;

FIG. 7 is a plan view of the air optimisation module of a data centrebuilding according to a second embodiment of the invention;

FIG. 8 is an end view of two adjacent rack storage areas according to asecond embodiment of the invention; and

FIG. 9 is a partial plan view of a data centre according to a thirdembodiment of the invention.

DETAILED DESCRIPTION

FIG. 2 shows a data centre building 10 according to a first embodimentof the invention.

The building 10 is rectangular with external walls 12. The building isdivided into front and rear sections by an internal dividing wall 12 a,located approximately one third of the length of the building from therear external wall.

The rear section (on the left in FIG. 2) defines an air optimisationroom 11, which provides a system of circulating cooling air in thebuilding 10. Ambient air 18 can enter the air optimisation room 11through an ambient air intake 13 in the rear external wall. Exhaust air16, which has been heated by IT equipment in the data centre, can enterthe air optimisation room through two exhaust air intakes 20 a and 20 bin the internal dividing wall 12 a. The ambient air intake 13 and theexhaust air intakes 20 a and 20 b are fitted with dampers so that theamount of ambient air and the amount of exhaust air entering the airoptimisation room may be controlled. When the ambient air intake damperis shut, no ambient air may enter the air optimisation room. Likewise,when the exhaust air intake dampers are shut, no exhaust air may enterthe air optimisation room.

Ambient air 18, exhaust air 16 or a mixture of ambient and exhaust aircan be treated/cooled in the air optimisation room and this air 18 a isthen used for cooling. If the ambient air outside the building 10 issufficiently cool, the ambient air may be used as cooling air, withoutrequiring any active cooling by the air optimisation room 11. Coolingair 18 a passes into the front section of the building 10 through twoair passages 17 in the internal dividing wall 12 a.

The front section (on the right in FIG. 2) of the building 10 defines arack room 19. The rack room 19 houses two rows of racks 14; one oneither side of the air passages 17. The racks 14 extend away from theinternal dividing wall 12 a, towards the front of the building. Althoughonly shown schematically in FIG. 2, there are 20 racks in each row, eachrack housing up to 40 items of IT equipment (typically server blades).There may therefore be a many as 1,600 items of IT equipment in theracks. A blanking panel 14 a extends between the front ends of the tworacks, thereby defining a cold region 19 a between the internal dividingwall 12 a, the two racks 14 and the blanking panel 14 a.

A hot region 19 b is defined on the other side of the racks 14 and theblanking panel 14 a. Air can escape from the hot region 19 b though ahot air exit 15 in the front external wall of the building. The hotregion and the cold region are made largely air tight through the use offurther blanking panels in empty racks and between the top of the racksand the ceiling of the rack room, so that air may only move from thecold region to the hot region through the servers, or through a duct 21.

The duct 21 is located between the top of one of the rows of racks 14and the ceiling. The duct 21 extends from the cold region to the hotregion. The end of the duct that meets the hot region is covered by ahinged flap (not shown), meaning that air may only enter the duct fromthe cold region and exit it into the hot region, and not vice versa. Theduct 21 contains an airflow velocity sensor (not shown).

In use, ambient air 18 enters the air optimisation room 11 through theambient air intake 13 and/or exhaust air enters the air optimisationroom through exhaust air intakes 20 a and 20 b. The air that enters theair optimisation room will from here on be referred to as supply air.The supply air can consist of just ambient air, just exhaust air, or amixture of ambient and exhaust air depending on the position of theambient air intake damper and the position of the exhaust air intakedampers. The supply air is cooled/treated as necessary in the airoptimisation room 11 and leaves through air passages 17 as cooling air18 a. The treatment and/or cooling of the supply air may be effected inaccordance with the second embodiment described below. The volume ofcooling air leaving the air optimisation room is controlled by avariable speed fan (not shown) in the air optimisation room.

The cooling air 18 a enters the rack room 19 into the cold region 19 a.The cooling air 18 a moves over the racks 14 in the rack room 19 toreach the hot region 19 b and in the process cools the racks 14. Theresulting hot air 16 coming off the racks 14 then leaves the rack roomthrough the hot air exit 15. If the exhaust air intake dampers 20 a and20 b are open then some of the hot air 16 will re-circulate back intothe air optimisation room 11.

The volume of air flow through the building is at least 12 m³s⁻¹. Such ahigh rate of supply of air is sufficient to cool the IT equipment in theroom via ambient air cooling alone for ambient air temperatures of up to24 degrees Celsius. The volume of air supplied to the cold region isadjusted according to the demand of the servers at any given time. Howthis adjustment is achieved will now be described.

If too much cooling air is being supplied to the cold region—i.e. thevolume of air flowing through passages 17 is greater than the volume ofair being drawn through the servers—the air pressure in the cold regionwill increase. As a result, the velocity of the air flowing through duct21 will increase. The velocity sensor in duct 21 provides velocityinformation to a control system that controls the speed of the variablespeed fan in the air optimisation room 11. When the velocity measured bythe velocity sensor is greater than 0.35 m/s, the control system reducesthe speed of the fan so that less air is supplied to the cold region.

If too little cooling air is being supplied to the cold region—i.e. thevolume of air flowing through passages 17 is less than the volume of airbeing drawn through the servers—the air pressure in the cold region willdecrease. As a result, the velocity of the air flowing through duct 21will decrease. When the velocity measured by the velocity sensor in duct21 falls below 0.35 m/s, the control system increases the speed of thevariable speed fan so that more air is supplied to the cold region.

FIG. 3 shows a second embodiment of the invention. In the embodimentshown in FIG. 3, there is an air optimisation room 120 located at therear of the building 100, a plant room 130 located in front of the airoptimisation room 120, a rack room 140 located in front of the plantroom 130, an above-floor hot air corridor 132 and an above-floor airsupply corridor 123.

The air optimisation room 120 contains an air optimisation unit 122, thelayout of which is shown in FIG. 6. The air optimisation unit 122 islocated adjacent the external right side wall of the data centrebuilding 100 so that an ambient air intake grille 121 on one end of theunit 122 lines up with an ambient air intake hole 113 in the externalwall of the data centre. The ambient air intake grille 121 includes adamper that is controllable so that the amount of air entering the airoptimisation unit 122 through grille 121 can be controlled.

The air optimisation unit 122 also has a second air intake in the formof a return air grille 125. The return air grille 125 is located at theright, front end of the optimisation unit 122, near the end wallincluding the ambient air intake grille 121. The return air grille 125includes a damper that is controllable so that the amount of airentering the air optimisation unit 122 through grille 125 can becontrolled.

The air optimisation unit 122 (shown in detail in FIG. 7) containsvarious air treatment apparatus, including a bank of variable speed fans700, air filters 710, an adiabatic cooler 720 and six DX cooling coils730. The air optimisation unit 122 also contains an air mixing box 740for mixing the air from return air grille 125 and ambient air intakegrille 121. The unit 122 also contains sound attenuation apparatus.

The wall between the plant room 130 and the air optimisation room 120includes several doors (not shown), through which the air treatmentapparatus may be accessed for maintenance. When the fans 700 areoperating at more than 35% of their maximum speed, the pressuredifferential between the air optimisation unit 122 and the plant room130 is such that it is very difficult to open these access doors. Sothat the air treatment apparatus may be inspected and maintained whilethe cooling system is in operation, there is a switch in the plant roomand switches next to each of the access doors on the air optimiser unitside.

When any of these switches is activated, the normal control process forthe fans 700 is overridden and their speed drops to a low level for 8seconds. This allows the door to be opened safely. After the 8 secondshas elapsed, the fans 700 then operate at the speed at which they wereoperating when the switch was pressed for 2 minutes, after which normalcontrol is resumed.

To the left side of the air optimisation unit 122 is an air supplycorridor 123. The air supply corridor 123 runs above-floor from the rearexternal wall, and along and in between the left side of the airoptimisation unit 122 and the left external side wall.

To the right side of the plant room 130 is a hot air corridor 132running above-floor along the width of the plant room 130 and along theexternal side wall of the building. The hot air corridor 132 alsoextends around the front of the plant room 130, in between the frontplant room wall and the rear side of the left-most row of racks. Thiscorridor also extends into the spaces 145 between the rear sides of theother rows of racks. This allows air from the rack room 140 to enter thehot air corridor 132.

On the left end wall of the plant room 130 is a plant room access door131. The door 131 allows access to the plant room 130 from the hot aircorridor 132.

The plant room 130 contains an air optimiser control panel 137, mountedon an internal side of the plant room walls. The air optimiser controlpanel 137 receives data from various sensors inside the building 100 andan outside ambient air temperature and relative humidity sensor TH₀.This outside ambient air temperature sensor TH₀ is placed outside thebuilding 100 near the ambient air intake grille 121. Sensor TH₀ actuallycomprises two separate temperature and humidity sensors, and thetemperature and humidity values provided to the air optimiser controlpanel 137 are averages of the values measured by the two sensors.

The air optimiser control panel 137 also receives data from three rackroom control panels (not shown), each of which is located at the end oneof the cold aisles 144.

The air optimiser control panel includes processors and memory forstoring pre-programmed instructions and control software. The airoptimiser control panel uses the information received from the sensorsand the rack room control panels together with the pre-programmedinstructions to control the fans, humidification apparatus, coolingsystem and the ambient air intake, exhaust and return dampers in orderto achieve effective cooling of the racks in the rack room 140.

The plant room 130 also contains fire suppression apparatus and a VESDA(Very Early Warning Smoke Detection Apparatus) fire detection monitoringpanel, mounted on an internal side of the plant room walls 134. Firesuppression gas discharge cylinders are connected to the airoptimisation unit 122 so that in the event of a fire (when the VESDAmonitoring panel is triggered), gas from the cylinders can be dischargedthrough the air optimisation unit 122 into air supply corridor 123.

The rack room 140 contains six elongate rectangular rack storage areas,the areas being parallel to each other. Hence, a passageway runningalong and in between the rear external wall of the building and the rearinternal wall 141 of the rack room 140 is defined. This passageway runsalong the width of the rack room 140 and is closed off from the rackroom area by the internal wall 141. The passageway forms part of the airsupply corridor 123.

Each rack storage area is effectively defined by a single row of racks143 running lengthways along the rack room 140, i.e. widthways acrossthe building, from the internal wall 141 to the rear end of the rackroom area. The rack storage areas are arranged in three pairs such thatwhen racks have been installed in the rack storage areas and the rackshave been filled with servers, each pair of rack storage areas includestwo rows of racks 143 arranged face-to-face. Between the front faces ofthe rows of racks making up each pair is a cold region in the form of anabove-floor cold aisle 144.

At the rear end of the rack room area, spanning across the ends of thetwo rack rows making up each pair, is a cold aisle blanking panel 147designed to close off the cold aisle 144 at the rear end. As shown byFIG. 8, above the front face of each row of racks 143 there areover-rack blanking plates 800 between the top of the racks and theceiling 810 of the rack room 140, which are designed to stop cold airtravelling over the racks 143. Blanking plates are also used to preventair flow through any other spaces between the racks and the floor,ceiling or ends of the cold aisle, as well as through spaces in theracks where there is no IT equipment. Above each row of racks there isalso a duct 150, 160, 170 joining each cold aisle 144 to each hot region145 between the back of the racks. These ducts 150, 160, 170 passthrough holes in the over-rack blanking plates 800. The end of each duct150, 160, 170 that joins the hot region 145 is covered by a hinged flap(not shown). Ducts 150, 160 and 170 are of rectangular cross-sectionmeasuring approximately 110 mm×54 mm. Each duct 150, 160, 170 containsan air flow velocity sensor (not shown), which provides air flowvelocity information to a rack room control panel (not shown). Hence,air can only leave the cold aisles 144 through the racks 143 and throughthe ducts 150, 160 and 170. Each cold aisle has its own rack roomcontrol panel, located on the cold aisle blanking panels 147.

Air from the supply air corridor 123 can enter the cold aisles 144through cooling air intake grilles 142, located on the internal wall 141(i.e. above-floor) in between the rows of racks 143. The grilles 142include dampers 182. Each damper 182 is controlled by the rack roomcontrol panel for its respective aisle so that a desired air flow regimecan be achieved.

The rack room control panels use proportional-integral (PI) controllersto control the dampers 182. A PI controller uses two separate parameterswhich may be thought of as the reaction to the current error, and thereaction to the accumulation of past errors. A weighted sum of these twoparameters is used to determine the actual reaction, for example how thedampers 182 move in response to a change in the airflow velocitymeasured by the sensors. The PI algorithm used by the controller istuned to provide optimal control of the dampers, such that the positionquickly stabilises at the optimal value and does not oscillate around itfor very long.

As shown by FIG. 4, each cooling air intake grille 142 is part of asecurable door 180 that can be opened and closed to allow personnelaccess from the air supply corridor 123 to the cold aisle 144 of therack room 140. Each cooling air intake grille door 180 is made fromaluminium and/or steel. Each door 180 opens by way of a hinge 184. Eachset of dampers 182 is connected to the rack room control panel for itsaisle by wiring which runs through a flexible tube 183.

On the front wall of the rack room 140, which is also the front externalwall of the building, is a hot air outlet grille 146. The grille 146 isdivided into four equally sized sections. Each of these sections has adamper that is individually controllable by the air optimiser controlpanel 137 so that the amount of hot air 16 that is exhausted from thebuilding 100 through hot air outlet grill 146 can be controlled.

The ambient air intake grille 113 is also divided into four equallysized sections, each of which has its own individually controllabledamper controlled by the air optimiser control panel 137. The airoptimiser control panel is programmed such that the dampers in the airintake grille 113 and the dampers in the exhaust grilles 146 move intandem and are always in the same position.

The air optimiser control panel 137 is programmed to open and shut theair intake and exhaust dampers in such a way that the finest possibledegree of control over the amount of air exiting and entering the datacentre is achieved. This means that, for example, if the ambient airintake dampers and the exhaust dampers are required to be 25% open, foreach grille one of the four sections will be fully open and the otherthree will be fully closed. If the cooling load increases from thissituation so that more airflow is needed, a second section will start toopen until the demand for cooling air is met. If the cooling loadincreases further, the second section will open more. If it reaches thefully open and the demand is still not met, a third section will startto open, and so on.

The air optimiser control panel 137 is programmed so that it instructsthe sections to open in a different order each week, cycling through allpossible orders. This evens out the wear on the actuators that move thedampers. The air optimiser control panel 137 uses PI controllers tocontrol the air intake and exhaust dampers.

In use, the data centre building 100 of FIG. 3 operates to cool theracks 143 in the rack room 140 by generating a sufficient quantity,velocity and pressure of cooling air 18 a in the air optimisation unit122. The operation of the air optimisation unit will now be described indetail, with reference to FIG. 7.

When the data centre is started up for the first time, the airoptimisation unit 122 must be turned on by activating a switch. Thiscauses the fans 700 to begin operating at a preset speed, withoutactivating any of the other air optimisation equipment. The fans 700operate at this preset speed for a preset time period of two minutes.During this period the air pressure across the fans is measured bypressure sensors upstream and downstream of the fans, and monitored bythe air optimiser control panel 137. If at the end of the two minuteperiod the pressure differential is greater or equal to 5 Pa, and hasbeen for at least 15 seconds, the rest of the air optimisation equipmentis activated and normal control as described below is established. Ifthe pressure differential condition is not met, then the rest of the airoptimisation equipment is not allowed to begin operation. Once normaloperation is established, the air optimisation unit functions asfollows.

Air entering the air optimisation unit may be ambient air 18, whichenters through ambient air intake grille 121, exhaust air 16, whichenters through return air intake grille 125, or a combination of ambientair 18 and exhaust air 16, depending on the positions of the ambient airintake damper and the return air intake damper (as explained above, theposition of the exhaust air dampers is always the same as the positionof the ambient air intake damper). The position of these dampers iscontrolled by the air optimiser control panel 137. The air optimisercontrol panel is connected to the outside ambient air temperature andrelative humidity sensor TH₀, which is located outside the building 100,near the ambient air intake hole.

Sensor TH₀ measures the temperature and relative humidity of the ambientair outside the building. The air optimiser control panel 137 uses thesemeasured values to determine which one of seven pre-defined zones on apsychrometric chart 600 the ambient air falls within. These zones areshown by FIG. 5. There are seven zones, referred to as zone 1, zone 2and so on, up to zone 7. In FIG. 5, zone 1 represents a relatively coldand dry zone, whereas zone 7 represents a relatively hot and humid zone.Further explanation concerning these seven zones and their function isprovided later on. Which zone the measured values fall within determineswhether the ambient air intake damper and the return air intake damperare open or closed, and hence whether the supply air consists of justambient air 18, just exhaust air 16, or a mixture of ambient air 18 andexhaust air 16. The exact position of each damper is controllable by theair optimiser control panel 137 using a PI controller.

If both the return air damper and the ambient air intake damper areopen, ambient air 18 and exhaust air 16 are mixed together in mixing box740 to create supply air.

From the mixing box, the supply air then passes through filters 710,which remove dust and other particles from the supply air. Once it haspassed through the filters 710, the supply air passes through a DXcooling system 730, which comprises six DX cooling coils and sixcondenser units. The DX cooling system 730 is controlled by the airoptimiser control panel 137 using a PI controller. Whether the DXcooling system is on or off, and if on, at what level it is operating,is determined by which zone the ambient air characteristics measured bysensor TH₀ fall within. The air optimiser control panel is programmed toonly allow operation of the DX system 730 when there is positive airflowacross the fans 700.

Once the supply air has passed through the DX cooling system 730 itpasses through the adiabatic cooler 720 and/or the bypass damper 750.When the bypass damper 750 is closed all of the supply air will passthrough the adiabatic cooler 720. When the bypass damper 750 ispartially open, some of the supply air will pass through the adiabaticcooler 720 and some will pass through the bypass damper 750. If theadiabatic cooler is operating at the same level in both scenarios, theair downstream of the adiabatic cooler 720 will be colder and more humidif the bypass damper is shut than if it is open.

The adiabatic cooler 720 consists of a matrix made of corrugated sheetsof glass fibre material. Water is supplied to the top of the matrix andflows down its corrugated surface. The supply air passes through airgaps between the sheets of moist material, picking up water vapour as itdoes so. This increases the relative humidity and lowers the temperatureof the supply air. The water that does not evaporate flows into astainless steel tank at the base of the unit before being re-circulatedup on to the matrix again. The matrix has four sections, each of whichhas a separate water supply that is individually controlled by the airoptimiser control panel. This allows the level of humidification andcooling provided by the adiabatic cooler to be varied between fourdifferent states. The amount of cooling can be further varied byaltering how much air passes through the bypass damper and therefore isnot cooled at all.

The amount of cooling done by the adiabatic cooler 720 is determined bywhich zone the temperature and relative humidity values measured bysensor TH₀ fall within. When the adiabatic cooler is on, the level atwhich it is operating at any given time is controlled in dependence onthe temperature and relative humidity measured by sensor TH₂. (Likesensor TH₀, sensor TH₂ actually comprises two separate temperature andhumidity sensors, and the temperature and humidity values provided tothe air optimiser control panel 137 are averages of the values measuredby the two sensors.) When the adiabatic cooler 720 is completely off(for example when the ambient air is in zone 7) the bypass damper 750 isfully open. When the adiabatic cooler 720 is on, the level at which itoperates and the exact position of the bypass damper 750 (which togetherdetermine the amount of cooling) are controlled by the air optimisercontrol panel 137 using a PI controller.

The bypass damper allows a much finer degree of control over the amountof cooling provided by the adiabatic cooler 720 than would be possiblewithout such a damper. When the adiabatic cooler is on, which (if any)of the water supplies for the adiabatic cooler 720 are on, and theposition of the bypass damper 750, at any given time is determined bythe air optimiser control panel based on the temperature and relativehumidity of the air immediately downstream of the adiabatic cooler asmeasured by sensor TH₂.

The control process for the adiabatic cooler 720 and the bypass damper750 will now be described. When no adiabatic cooling is required, allfour sections of the adiabatic cooler are off (i.e. no water is flowingthrough them) and the bypass damper 750 is fully open. If the coolingdemand increases, one section of the adiabatic cooler is brought onlineusing the following procedure, which is programmed in to the airoptimiser control panel 137.

When a new section is about to be brought online, the bypass damper isalways instructed to move to the fully closed position. When the bypassdamper has been fully closed for 30 seconds, the new section isactivated by turning on the water supply to that section. Once the watersupply has been switched on, the bypass damper is moved to a pre-setposition corresponding to approximately two-thirds open. Once thisactivation sequence has been completed, the amount of cooling is thenadjusted to exactly meet the demand at any given time by adjusting theposition of the damper. If the cooling demand increases to the pointwhere it is not met even with the damper fully closed, an additionalsection of the adiabatic cooler must be brought online.

The air optimiser control panel is programmed such that there is alwaysa delay of 30 seconds between the bypass damper 750 closing and anadditional section of the adiabatic cooler 720 switching on. This isbecause the inactive sections of the cooler may still be wet fromprevious use, in which case more cooling than expected would result whenthe damper was fully closed. If the cooling demand is still not metafter the 30 seconds has passed, a second section of the adiabaticcooler 720 is switched on and the damper is moved to the pre-setposition. As cooling demand increases, this process will be repeateduntil all four sections of the adiabatic cooler are switched on and thebypass damper 750 is fully closed, at which point the adiabatic cooler720 is providing its maximum amount of cooling.

As the cooling load decreases, the bypass damper 750 is opened. If thedamper reaches the fully open position and there is still too muchcooling being done, a section of the adiabatic cooler is switched off(by shutting off its water supply) and at the same time the bypassdamper 750 is instructed to start closing. The bypass damper takesapproximately 2.5 minutes to move from fully open to fully closed. Asection of the cooler will become completely dry approximately 5-10minutes after it has been switched off if there is airflow through it.

As the recently switched off section dries, the amount of coolingprovided by the adiabatic cooler drops. The bypass damper is instructedto close to compensate for this drop. The bypass damper 750 will notusually reach the fully closed position, instead it will stop closingonce the data from sensor TH₂ show that the amount of cooling beingprovided meets the cooling demand and hence the cooling air meets thepre-defined criteria for cooling air programmed into the memory of theair optimiser control panel 137.

If the cooling demand drops further, the bypass damper 750 will open. Ifit reaches fully open and the amount of cooling being done is still toohigh, another section of the cooler will be switched off using theprocess described above. The degree of control provided by thecombination of the four-sectional adiabatic cooler 720 and the bypassdamper 750 is such that the amount of adiabatic cooling being done canbe tailored to closely match the cooling demand at any given time.

Once the supply air has passed through the adiabatic cooler 720 or thebypass damper 750, its relative humidity will meet pre-defined criteriafor cooling air programmed into the memory of the air optimiser controlpanel 137 but its temperature will be approximately 1° C. too low tomeet the pre-defined criteria for cooling air. This is because thetemperature of the supply air increases by approximately 1° C. as itpasses through the fans 700.

The temperature and relative humidity of the cooling air supplied to theIT equipment is measured by sensor TH₂. If the temperature measured byTH₂ deviates from the pre-defined criteria for cooling air by more than5° C. then an alarm is raised.

The pre-defined criteria for cooling air are represented by a singlecontrol line 610 on psychrometric chart 600 (see FIG. 5), which inaccordance with convention, has a horizontal axis measuring dry-bulbtemperature and a vertical axis measuring moisture content. The user canselect an appropriate control line based on the characteristics of theIT equipment installed in the data centre. In the current example thecontrol line joins point 620 (representing a temperature of 18° C. and arelative relative humidity of 45%), point 630 (representing atemperature of 24° C. and a relative humidity of 65%) and point 640(representing a temperature of 24° C. and a relative humidity of 80%).The cooling air supplied to the IT equipment may have temperature andrelative humidity substantially equal to any point on this line. Eachsection of the control line 610 is defined by the equation of a straightline having terms for enthalpy, moisture content and dry bulbtemperature.

The psychrometric chart 600 is divided into a plurality of differentzones around the control line. The zones are bounded by the control line610, lines of constant enthalpy, lines of constant temperature and/orlines of constant moisture content. The boundaries of the zones arecalculated by the control process panel 137 when the user defines thecontrol line that is to be used. In the current example the control line610 has two straight line sections, which leads to seven zones. FIG. 6shows an alternative control line 680 which has three straight linesections. The control line 680 in FIG. 6 results in nine separate zones,since zone 1 has been split into three separate zones 1 a, 1 b and 1 c.The control line 680 of FIG. 6 has the effect of reducing the waterconsumption of the data centre as compared with control line 610 of FIG.5, since it allows the supply air provided to the IT equipment to beless humid. The control line can be altered while the data centre is inoperation, in which case the control process panel 137 willautomatically recalculate the number, size and position of the zones.

The sensor TH₀ measures the temperature and relative humidity of theambient air 18, and the moisture content and enthalpy of the ambient air(hereafter “measured moisture content” and “measured enthalpy”) arecalculated from these values as measured. A processor in control processpanel 137 uses the measured moisture content and measured enthalpyand/or the measured temperature and measured relative humidity todetermine which zone the ambient air falls within. If the ambient air 18falls within zones 1, 3, 5, 6 or 7 then the control process panel 137can determine the zone by comparing the measured temperature, measuredmoisture content and/or measured enthalpy values with the constantvalues of temperature, moisture content and/or enthalpy that representthe zone boundaries. For example, if the measured temperature is greaterthan 24° C. and the measured enthalpy lies between the valuesrepresented by the upper and lower lines of constant enthalpy that boundzone 5, the outside air will have characteristics that fall within zone5 (there will be no need for comparison of other values). If the ambientair 18 falls within zone 2 or zone 4, the control process panel 137 mustadditionally determine which side of the control line the ambient air 18lies by substituting the measured temperature into the equation of thecontrol line. If the resulting value for moisture content is greaterthan the measured moisture content then the ambient air is in zone 4. Ifthe resulting value is less than the measured value, then the ambientair 18 is in zone 2.

The psychrometric characteristics of humid air vary with air pressure.As such, the control process panel 137 is able to compensate (orrecalibrate) to account for atmospheric air pressure. Whilst day-to-dayvariations need not be accounted for, the average atmospheric airpressure of ambient air at a given location, dictated for example byaltitude, may usefully be taken into account. The control process panel137 is pre-programmed with the altitude at which the data centre islocated and thus adjusts its calculations accordingly.

Once it has been determined which zone the ambient air falls into, theprocessor in control process panel 137 calculates the most efficient wayto adjust the temperature and relative humidity of the ambient air 18(i.e. using the most amount of free cooling and/or the least amount ofmechanical cooling possible) until it is somewhere on the control line610, and selects a target point on the control line accordingly. Thus atarget point (or set point) is calculated on the basis of thepsychrometric characteristics of the outside air, such that the targetpoint lies at a position on a control line so that the target point canbe reached in an energy efficient manner.

Table 1 shows how the air optimisation unit 122 adjusts the temperatureand relative humidity of the ambient air 18 to be on the control linedepending on which zone the ambient air 18 falls into.

When the ambient air 18 is in zone 1, which will be a significant amountof the time in the UK, it will be adjusted so that its temperature andrelative humidity are substantially equal to a point on the control line610 by mixing it with exhaust air 16, adiabatically cooling theresulting supply air, and then passing it through fans 700. For example,for ambient air having temperature and relative humidity represented bypoint 650 on the psychrometric chart 600, the air optimiser controlpanel 137 calculates a first target temperature and relative humidityfor the mixed supply air, this first target being represented by point660 on chart 600. The first target point 660 is on the boundary linebetween zones 1 and 4, since this is the first line of constant enthalpy(the diagonal lines on the chart 600) to intersect the control line 610.

The air optimiser control panel 137 then modulates the position of theintake, exhaust and return dampers (the position of the intake andexhaust dampers is always the same), based on the temperature andrelative humidity measured by a sensor TH₁ located between the DXcooling coils 730 and the adiabatic cooler 720, until the valuesmeasured by sensor TH₁ are equal to the first target values 660 for themixed supply air. The air optimiser control panel also receivesinformation about the temperature and relative humidity of the exhaustair 16 from sensor TH₃, which is located near return air intake grille125.

The temperature and relative humidity of air can be adjusted along thelines of constant enthalpy without any energy needing to be input intothe system. The temperature and relative humidity of the mixed supplyair can therefore be adjusted from point 660 to a point approximately 1°C. colder than the final target point 670 on the control line byadiabatic cooling (since this does not change the enthalpy of the air),which uses very little energy. Passing the air through fans 700 thenraises its temperature by approximately 1° C., so its characteristicsare then equal to the final target point 670.

The air optimiser control panel 137 modulates the level at which theadiabatic cooler 720 is operating and the position of the bypass damper750, using PI controllers, until the values measured by sensor TH₂ aresubstantially equal to the final target values 670. Supply air 18 adownstream of fans 700 therefore meets the pre-defined criteria forcooling air to be supplied to the IT equipment. The air optimisercontrol panel 137 is programmed to accept a greater tolerance in therelative humidity of the cooling air than in the temperature of thecooling air. For example if the relative humidity measured by sensor TH₂is up to 10% higher or lower than the final target value represented bypoint 670, it will be deemed to be at this target value. On the otherhand, the temperature measured by sensor TH₂ will be deemed to be at thefinal target value for cooling air if it is up to 0.5% higher or lowerthan the target value.

The same control principles apply when the ambient air is in the otherzones, so these will be discussed more briefly.

When the ambient air 18 is in zone 2, it can be adjusted to a statewhere its temperature and relative humidity lie on the control line 610purely by mixing with exhaust air 16, without any adiabatic cooling. Thetarget value for temperature is calculated by substituting the measuredmoisture content of the ambient air 18 into the equation of the controlline between points 670 and 630 and solving this equation to give therequired temperature for the supply air (i.e. the cooling air that issupplied to the IT equipment to be cooled). The moisture content of theambient air 18 and exhaust air 16 will usually be approximately the samewhen the ambient air is in zone 2, so the relative humidity is notchanged by the mixing process. Whilst in theory the target temperaturecan be achieved solely by mixing exhaust and ambient air, in practicewhen the ambient air characteristics are very close to the boundary withzone 4, some adiabatic cooling is often required, and this is allowed bythe programming of the control process panel 137.

When the temperature and relative humidity of the ambient air are suchthat it falls into zone 3, the ambient air 18 can be adjusted to a statewhere its temperature and relative humidity lie on the control line 610purely by mixing with exhaust air 16. Where the control line bounds zone3 it is a line of constant temperature, so there is no need for thecontrol process panel 137 to calculate a target temperature (because thetarget temperature is that represented by the control line dividingzones 3 and 5). As with zone 2, the moisture content of the ambient airwill be substantially unchanged.

When the temperature and relative humidity of the ambient air 18 aresuch that it falls into zone 4, the ambient air may be adjusted to astate where its temperature and relative humidity lie on the controlline 610 purely through adiabatic cooling. The target value fortemperature is calculated by substituting the measured enthalpy of theambient air 18 into the equation of the control line between points 670and 630 and solving this equation to give the required temperature forthe supply air. The relative humidity of the supply air will be greaterthan the relative humidity of the ambient air as a consequence of usingadiabatic cooling, which conserves the enthalpy of the air.

When the ambient air falls into zone 5, the ambient air can be adjustedto a state where its temperature and relative humidity lie on thecontrol line 610 using just adiabatic cooling. Where the control linebounds zone 5 it is a line of constant temperature, so the controlprocess panel 137 simply sets the target temperature at this constanttemperature. As with zone 4, the relative humidity of the ambient airwill be increased by the adiabatic cooling process.

When the ambient air is in zones 4 or 5 there is no advantage gained bymixing the ambient air 18 with exhaust air 16, so the air optimisercontrol panel 137 closes the return air damper and fully opens theambient air intake damper (and the exhaust damper).

When the temperature and relative humidity of the ambient air are suchthat it falls into zone 6, there is also no advantage to mixing theambient air 18 with exhaust air 16. In this situation the supply air,which consists entirely of ambient air 18, must be cooled using DXcooling coils 730 until its temperature and relative humidity are at apoint on a line of constant enthalpy that intersects the control line610 (i.e. a line of constant enthalpy that intersects the line at ornear point 640). The DX cooling system 730 includes condensers as wellas cooling coils, and can operate in either a pure cooling mode or in acombined cooling and dehumidification mode. Once the supply air is atsuch a point it is cooled adiabatically until its characteristics aresubstantially equal to point 640 on the control line 610 (taking intoaccount the heating caused by the fans).

TABLE 1 Types of cooling according to zone Composition DX AdiabaticBypass Zone of supply air cooling cooler damper TH1 controls TH₂controls 1 Mixture Off On Modulating Intake and Adiabatic return dampercooler and positions bypass damper 2 Mixture Off Off Open — Intake andreturn damper positions 3 Mixture Off Off Open — Intake and returndamper positions 4 100% Off On Modulating — Adiabatic ambient air coolerand bypass damper 5 100% Off On Modulating — Adiabatic ambient aircooler and bypass damper 6 100% On On Modulating Rate of DX Adiabaticambient air cooling cooler and bypass damper 7 100% On Off Open Rate ofDX Rate of DX exhaust air cooling cooling

When the temperature and relative humidity of the ambient air are suchthat it falls into zone 7, the conditions of the ambient air 18 are suchthat it requires more energy to adjust them to lie on control line 610than the amount of energy required to adjust the characteristics ofexhaust air 16 to lie on the control line. As such, the air optimisercontrol panel 137 instructs the data centre to operate in a fullrecirculation mode in which the ambient air intake damper and theexhaust damper are closed and the return air damper and the adiabaticcooler bypass damper 750 are fully open, meaning that the supply airconsists entirely of exhaust air 16. In this situation the supply airmust be adjusted so that its temperature and relative humidity are onthe control line 610 (taking into account the heating caused by thefans) using only DX cooling and dehumidification.

When the characteristics of the ambient air 18 change such that it movesfrom one zone to another, the control process panel 137 determines whichzone the air has passed from and which zone it has passed into and usesthis information to determine what the air optimiser needs to do toensure that the temperature and relative humidity of the supply airremain on the control line 610.

When the zone changes into zone 1 from zone 2 or zone 4, the controlprocess panel performs a proportional calculation based on enthalpy. Theenthalpy of the exhaust air 16, the enthalpy of the ambient air 18 andthe enthalpy of the required set point (point 620) are used to calculatethe proportions of exhaust and ambient air that must be combined toachieve the set point enthalpy. This in turn determines the position ofthe intake, exhaust and return air dampers and the control process panel137 moves the dampers to the positions so calculated and keeps themthere for a pre-set period of three minutes. After this period haselapsed, the position of the dampers is once again allowed to vary alongwith the characteristics of the ambient air 18.

When the zone changes into zone 2 or zone 3 from any other zone, thecontrol process panel 137 performs a proportional calculation based ontemperature. The temperature of the exhaust air 16, the temperature ofthe ambient air 18, and the temperature of the required set point areused to calculate the proportions of exhaust and ambient air that mustbe combined to achieve the set point temperature. The positions of theintake, exhaust and return air dampers are adjusted accordingly and heldin the calculated positions for three minutes.

When the zone changes into zones 4 to 7 there is no mixing of ambientand exhaust air so the damper positions do not need to be calculated.

The bank of variable speed fans 700 delivers cooling air 18 a into thecold air corridor 123 at a rate determined by the air optimiser controlpanel 137 based on information on the air flow velocities measured bythe velocity sensors 150, 160 and 170 and the position of the dampers inthe cooling air intake grilles 142 a, 142 b and 142 c communicated to itby each of the rack room control panels.

The speed at which the fans can operate is limited by a maximum speedcalculated by the air optimiser control panel 137 every 10 seconds. Thismaximum speed is twice the speed necessary to deliver the theoreticalmaximum amount of cooling air required by the IT equipment installed inthe data centre at a particular time, as calculated based on the actualpower consumption of the IT equipment at that time. The air optimisercontrol panel 137 receives values for current power usage of the ITequipment at one minute intervals. In the event that one or more of thevelocity sensors 150, 160 and 170 provides an artificially low readingto the air optimiser control panel 137, for example because baffles havebeen removed in a cold aisle for maintenance purposes, then the pre-setmaximum fan speed prevents the variable speed fans 700 from operating atan unnecessarily high level. This consequently improves the energyefficiency of the data centre. However, during normal operation of thedata centre the fan speed should always be significantly lower than thismaximum limit.

The cooling air 18 a is pushed out of the air optimisation unit 122 andmoves above-floor along the air supply corridor 123. The dampers in thecooling air intake grille(s) 142 are controlled in combination with thespeed of the variable speed fans 700 so as to ensure that exactly theright amount of cooling air 18 a is supplied to each cold aisle 144 tomeet the demand of the servers facing into that aisle at any given time.The cooling air 18 a is drawn through the servers by their integralfans, and cools them as it goes through.

The resulting hot air 16 moves above-floor through the hot aisles 145 inthe rack room 140 to the hot air corridor 132. The pressure differentialbetween the cooling air 18 a and the hot air 16 is maintained at asufficient level to ensure there is no return of hot air 16 through theracks. This is done by monitoring the amount of air flowing throughducts 150, 160 and 170 using the air flow velocity sensors located inthese ducts. The air flow velocity information for each cold aisle isfed to the rack room control panel for that aisle. The rack room controlpanel continuously passes this information to the air optimiser controlpanel 137 in the plant room 133, together with information about theposition of the cooling air intake damper for the aisle in question.

The air optimiser control panel 137 and the rack room control panels arepre-programmed with a set point for the velocity of air flowing throughducts 150, 160 and 170. In the present example the set point is 0.35m/s. If the velocity measured by any of the velocity sensors drops belowthis set point, the rack room control panel for the cold aisle supplyingthe duct in which that sensor is located will increase the amount of airbeing delivered to that aisle by opening the cooling air intake damperof that aisle further. If the damper reaches the fully open position andthe airflow velocity measured by the sensor is still below the setpoint, the rack room control panel for that cold aisle will send arequest to the air optimiser control panel 137 to increase the speed ofthe variable speed fans 700 until the velocity measured by the sensor isat the set point.

If this increase in the speed of the fans 700 causes the airflowvelocity measured by any of the other sensors to rise above the setpoint, the rack room control panels for the aisles supplying theaffected sensors will reduce the degree of openness of the cooling airintake dampers for those aisles until the airflow velocity measured byeach sensor is at the set point. If the point is reached where one ormore of the aisles has a fully closed damper and the air flow velocitymeasured by the sensor for that aisle is still measuring a velocitygreater than the set point, the rack room control panel for the coldaisle supplying the duct in which that sensor is located will send arequest to the air optimiser control panel 137 to reduce the speed ofthe variable speed fans 700 until the airflow velocity measured by thatsensor is at the set point.

If at any time the airflow velocity measured by all of the velocitysensors is above the set point, the control process panel 137 willreduce the speed of the fans 700 until the airflow velocity measured byat least one of the sensors is at the set point.

The control system just described is designed so that the variable speedfans 700 are always operating at the minimum speed at which it ispossible to meet the demand of the aisle having the largest cooling loadat any given time. This significantly reduces the energy used by thedata centre.

The air optimiser control panel 137 and the rack room control panels areall programmed with a watchdog routine to automatically monitor thehealth of the communications network linking the panels. Every 10seconds, each of the rack room control panels sends a value of 100 tothe air optimiser control panel 137, which returns a value of 0 to therack room control panels 10 seconds later. This means that when thecommunications network is operating correctly, the values registered byeach of the panels will oscillate between 100 and 0 every 10 seconds.

If a link between one of the rack room control panels and the airoptimiser control panel is broken, that rack room control panel willregister a constant value of 0 or 100. If the value does not change for90 seconds the rack room control panel will go into a failsafe mode inwhich it fully opens the cooling air intake dampers for its aisle. Ifthere is a fault with the rack room control panel itself, the coolingair intake dampers for that aisle are set up to move to a defaultposition of fully open.

In this situation the air optimiser control panel will also register aconstant value for the rack room control panel in question. If thisvalue does not change for 90 seconds the air optimiser control panelwill go into a fault mode in which the data centre operates in fullrecirculation mode using only the DX cooling system, with the fans 700running at the calculated maximum speed limit based on the IT equipmentpower consumption and all of the cooling air intake dampers for the coldaisles fully open. In the fault mode the temperature of the supply airis controlled to a fixed pre-set temperature of 22° C. This fault modeensures that sufficient cooling air is provided to all the servers ifthe air optimiser control panel is unable to determine the cooling loadbeing experienced by one or more of the aisles because of a failure ofthe communications network or a fault with one or more of the rack roomcontrol panels. There is a backup air optimiser control panel whichtakes over in the case of a fault with the main air optimiser controlpanel. The data centre can only operate in the fault mode describedabove if it is being controlled by the backup air optimiser controlpanel.

In the event of the VESDA (Very Early Warning Smoke Detection Apparatus)system detecting smoke, as an initial step, the air optimisation controlpanel 137 causes air flow into and/or out of the building to cease byinstructing the air optimisation control panel to close the dampers inthe hot air outlet grille 146 and the ambient air intake grille 121. Thebuilding is therefore operating in full re-circulation mode, in whichall of the cooling is done by the DX system 730.

If smoke is no longer detected then it is likely that any fire isoutside the building and so there is no need to release the firesuppression gas. If, on the other hand, smoke is detected by twodifferent types of smoke detector, it is likely that there is a firewithin the building and fire suppression gas will be released.

As the fire suppression gas rapidly expands, release vents (not shown)in the building 100 are activated to maintain the building integrity.

When the building is in recirculation mode because the VESDA system hasbeen activated, all of the cooling for the servers is provided by DXcooling coils 730. Since it is likely that the DX coils will have beenoff at the time of the alarm, as soon as the building goes intorecirculation mode the air optimiser control panel 137 reduces thetarget temperature for the cooling air to 12° C. so that the coolingcoils 730 will start up quickly. The air optimisation control panel 137then increases the target temperature by 1° C. every minute until itreaches 22° C. This ensures that the temperature of the cooling air doesnot become too high whilst the building management system is adjustingto the new mode of operation.

If it turns out that the activation of the VESDA system was a falsealarm, the air optimiser control panel 137 will switch the building fromfull re-circulation mode to the standard mode of operation based on theconditions of the ambient air. When the ambient air falls into zones 4,5 and 6 this means that the building goes into a mode where it uses 100%ambient air for cooling. The ambient air may require a significantamount of adiabatic cooling in order to adjust its temperature andrelative humidity to meet the criteria for cooling air. During the timethe building was in re-circulation mode the adiabatic cooler 720 willhave been offline and its matrix will have completely dried out. Ittakes approximately 10 minutes to bring the adiabatic cooler online tofull capacity from this situation.

Clearly, if the switch from re-circulation mode into the normal controlmode for zones 4, 5 or 6 was made without allowing for this, theadiabatic cooler would not at first be able to provide the amount ofcooling required and the temperature of the supply air would become toohigh. To avoid this scenario, the air optimiser control panel isprogrammed to switch on the adiabatic cooler (all four sections if theambient air is in zone 4 or 5, two sections if the ambient air is inzone 6) and operate at this fixed level for two minutes before the levelof operation of the DX coils is reduced. This allows the matrix of theadiabatic cooler 720 to become sufficiently wet to cope with the coolingrequirements of the ambient air. The level of operation of the adiabaticcooler is then fixed for a further minute before automatic control isrestored.

As part of the procedure for returning to normal operation, a damperposition calculation is carried out using the calculated supplytemperature setpoint, outside air temperature and return air temperatureto determine what position the exhaust, intake and return air dampersshould be in to maintain the current calculated setpoint. If the ambientair is in zones 2 or 3, then the temperature based calculated damperposition is used for 3 minutes with the dampers held at a position 5%lower than the actual calculated demand. The same procedure is used ifthe ambient air is in zone 1, but the damper position is calculatedbased on enthalpy instead of temperature. This offset encourages theautomatic demand to wind up to the correct level so that after 3 minuteswhen the artificial signal is removed the dampers will automaticallycontrol to the right position.

The air optimiser control panel 137 will also switch the data centreinto the full recirculation mode where all cooling is provided by the DXsystem 730 if it detects a fault with the adiabatic cooler 720. The samecontrol processes for entering and leaving the full recirculation modeare used as described above.

In the event that, while the building is in full re-circulation modebecause the VESDA system has detected smoke, but not an actual fire, twoor more of the DX cooling coils 730 fail and the temperature of thecooling air measured by sensor TH₂ becomes too hot to meet thepre-defined criteria for cooling air, the air optimiser control panel137 will switch the building into a mode where the cooling air comprises100% ambient air to ensure that the cooling demand of the servers ismet. If a fire inside the building is subsequently detected, the airoptimiser control panel 137 will switch the building back into fullre-circulation mode in spite of the failed DX coils, and will releasethe fire suppression gas. At this point it is more important toextinguish the fire than to meet the cooling demand of the servers.

FIG. 9 shows a data centre according to a third embodiment of theinvention. This data centre is the same as the data centre of the secondembodiment with an above-floor air supply corridor 923 and a hot aircorridor 932; however, it has a second air optimisation unit 920 locatedat the opposite end of the rack room to the first air optimisation unit910. The contents and operation of the second air optimisation unit 920are identical to that of the first air optimisation unit 910, which aredescribed above in relation to the second embodiment. There is also anadditional plant room 940 between the second air optimisation unit 920and the rack room.

Each air optimisation unit is controlled by its own dedicated airoptimiser control panel. The first air optimiser control panel 937 islocated in the first plant room 930 and the second air optimiser controlpanel 938 is located in the second plant room 940. Each control panelhas its own backup control panel (not shown). Aspects of the operationof the data centre of the third embodiment which differ from the datacentre of the second embodiment will now be described. Any aspects notmentioned may be assumed to be in accordance, mutatis mutandis, with thedescription provided above in relation to the second embodiment.

The two air optimiser control panels 937, 938 are connected via acommunications link and each is programmed to recognise and monitor thepresence and activity of the other. Each air optimiser control panelreceives data from its own set of temperature and humidity sensors (onlyTH₂ a and b are shown) since the temperature and humidity conditions maybe different between the two ends of the data centre. Each control panel937, 938 therefore calculates a set point for the temperature andhumidity of the cooling air independently, and may perform differentcooling processes to achieve the set points.

By contrast, both air optimiser control panels 937, 938 receive air flowvelocity data from all of the velocity sensors in the rack room, and socooling air is always supplied at the same rate from each airoptimisation unit 910, 920 and both air optimiser control panels alwayssend the same control signals to the aisle control panels (not shown)which control the degree of openness of the vents between the air supplycorridor 923 and the cold aisles 944.

If one of the air optimiser control panels fails, its associated backupcontrol panel will start up. The other air optimiser control panel willregister the fault and will also hand over control to its backup controlpanel. This ensures that both air optimisation units 910, 920 arerunning in fault mode. Clearly if one unit was operating in fault mode(where all air is recirculated, a fixed set point is used and all thecooling is provided by the DX system) and the other unit was operatingnormally, the two units could end up operating in conflict with eachother. Like the main air optimiser control panels, the backup panels areconnected via a communications link and each is programmed to recogniseand monitor the presence and activity of the other. The back up panelsare programmed so that if one is activated, it instructs the other toactivate as well.

If the adiabatic cooler in either of the air optimisation units 910, 920fails, this will be registered by both air optimiser control panels 937,938 and both of the air optimisation units will be switched into faultmode.

Whilst the present invention has been described and illustrated withreference to particular embodiments, it will be appreciated by those ofordinary skill in the art that the invention lends itself to manydifferent variations not specifically illustrated herein. By way ofexample only, certain possible variations will now be described.

It will be seen that in the above embodiments, the air is supplied tothe cold region/cold aisle by means of an over-floor passageway, forexample the personnel corridor (air supply corridor 123) from the sourceof cooling air (the air optimisation unit 122) to the rack room 140. Asan alternative, the air supply corridor from the air optimisation unit122 to the rack room 140 may be independent of the personnel passagewayleading to the rack room module(s) 140. Instead of supplying air abovethe floor as in the illustrated embodiments, the air may be supplied atleast partially via an under-floor duct.

Cooling air could be transported into the cold aisles through a wall ofthe rack room via one or more apertures or passageways in the wall thatare not arranged to permit personnel access. There may be an access doorto the rack room that is not part of the intended path for cooling air.

The ducts 150, 160 and 170 need not be located above the rows of racks,instead they may be located between adjacent racks, below the rows ofracks, or within a rack or racks. The ducts may not have a hinged flappreventing air entering from the hot end, instead the velocity sensorsin the ducts may be directional velocity sensors which provideinformation about the direction of the air flow.

There may be more than one duct connecting each cold aisle 144 to anadjacent hot aisle 145. In this case each duct would contain a velocitysensor that provides a velocity reading to the air optimiser controlpanel, but only the reading from one of the sensors would be used by theair optimiser control panel 137. The air optimiser control panel woulduse only the lowest of the two velocity readings. The other sensor wouldserve as a backup in case the first sensor failed.

The arrangement of the air optimisation unit 122 may be different fromthat described above. For example the DX cooling coils 730 may bedownstream of the humidifier 720.

The control line 610 may be in the form of a closed loop or a point. Thenumber and shape of the zones will vary according to the length andconfiguration of the control line.

Individual aspects of one embodiment may have application in otherembodiments of the invention. For example, the use of a velocity sensormay have independent application of the method of controlling coolingair to meeting pre-set criteria. The treatment and/or cooling of thesupply air to supply cooling air mentioned in the first embodiment neednot therefore be effected in accordance with the method set out in thesecond embodiment. For example, the controlling of the temperature andhumidity of the cooling air produced from the supply air may consist ofensuring that the cooling air is between relative wide acceptable limitsof temperature and/or humidity.

The rack room control panels could instead useproportional-integral-derivative (PID) controllers to control thedampers 182. A PID controller uses three separate parameters which maybe thought of as the reaction to the current error, the reaction to theaccumulation of past errors, and the reaction based on a prediction offuture errors. A weighted sum of these three parameters is used todetermine the actual reaction, for example how the dampers 182 move inresponse to a change in the airflow velocity measured by the sensors.The PID algorithm used by the controller is tuned to provide optimalcontrol of the dampers, such that the position quickly stabilises at theoptimal value and does not oscillate around it for very long.

Where in the foregoing description, integers or elements are mentionedwhich have known, obvious or foreseeable equivalents, then suchequivalents are herein incorporated as if individually set forth.Reference should be made to the claims for determining the true scope ofthe present invention, which should be construed so as to encompass anysuch equivalents. It will also be appreciated by the reader thatintegers or features of the invention that are described as preferable,advantageous, convenient or the like are optional and do not limit thescope of the independent claims. Moreover, it is to be understood thatsuch optional integers or features, whilst of possible benefit in someembodiments of the invention, may not be desirable, and may therefore beabsent, in other embodiments.

The invention claimed is:
 1. A method of cooling a data centre withcooling air, wherein the method comprises the following steps: (a)defining criteria for the temperature and humidity of the cooling air,wherein the criteria correspond to a region on a notional psychrometricchart, said region spanning a range of temperatures on the notionalpsychrometric chart and spanning a range of humidities on the notionalpsychrometric chart; (b) determining the psychrometric characteristicsof ambient air from outside the data centre, the characteristicscorresponding to a single point on the notional psychrometric chart; (c)selecting a single set point for the psychrometric characteristics ofthe cooling air from multiple possible set points spanning a range oftemperatures on the notional psychrometric chart, the multiple possibleset points being in said region on the psychrometric chart, the selectedsingle set point being chosen in dependence on the temperature and onthe humidity of the ambient air; (d) producing cooling air havingpsychrometric characteristics substantially corresponding to the singleset point set so selected; and (e) delivering the cooling air to aregion in the data centre to be cooled; wherein, the criteria for thetemperature and humidity of the cooling air are stored in memory of aprogrammable controller, and wherein step (c) is performed using aprogram run on the programmable controller, the program using thedetermined psychrometric characteristics of step (b).
 2. The methodaccording to claim 1, wherein all such multiple possible set points lieon a line that lies in said region on the psychrometric chart, said linespanning a range of temperatures on the notional psychrometric chart andspanning a range of humidities on the notional psychrometric chart. 3.The method according to claim 2, wherein the line on which all suchmultiple possible set points lie is a single line of finite length,which starts at one point on the notional psychrometric chart and endsat a different point.
 4. The method according to claim 3, wherein thepoint at which the single line of finite length starts is at both adifferent temperature and at a different humidity than the point atwhich the single line of finite length ends.
 5. The method according toclaim 4, wherein all points on the single line of finite lengthrepresent set-points that it is possible, during performance of themethod, to select during step (c).
 6. The method according to claim 3,wherein the line on which the multiple possible set points lie comprisesmultiple straight line sections on the notional psychrometric chart. 7.The method according to claim 6, wherein the multiple straight linesections consist of either two straight line sections joining threepoints on the notional psychrometric chart or three straight linesections joining four points on the notional psychrometric chart.
 8. Themethod according to claim 7, wherein the point at which the single lineof finite length starts is at both a different temperature and at adifferent humidity than the point at which the single line of finitelength ends.
 9. The method according to claim 6, wherein there aremultiple ways in which the cooling air is produced and delivered, eachsuch way corresponding to a unique mode of operation and a zone on thenotional psychrometric chart, each unique start or end point of each ofthe multiple straight lines defining in part the extent of at least oneof the unique zones.
 10. The method according to claim 9, wherein atleast one of the zones is defined by a line of constant enthalpy thatterminates at the start or end point of one of the multiple straightlines.
 11. The method according to claim 9, wherein at least one of theunique zones is defined by a line of constant moisture content thatterminates at the start or end point of one of the multiple straightlines.
 12. The method according to claim 9, wherein each of the zones isdefined at least in part by a line of constant enthalpy or of constantmoisture content that terminates at the start or end point of one of themultiple straight lines.
 13. The method according to claim 9, whereinthe method includes selecting a set point for the psychrometriccharacteristics of the cooling air during a first unique mode ofoperation, the set point corresponding to a first temperature andsubsequently selecting a second set point for the psychrometriccharacteristics of the cooling air without changing from the firstunique mode of operation, the set point corresponding to a secondtemperature different from the first.
 14. The method according to claim6, wherein at least one of the multiple straight line sections on thenotional psychrometric chart is a line of constant temperature.
 15. Themethod according to claim 1, wherein the step of selecting the singleset point is carried out such that producing cooling air withpsychrometric characteristics corresponding to the chosen set pointrequires no more energy than producing cooling air with psychrometriccharacteristics corresponding to any of the other multiple possible setpoints.
 16. The method according to claim 1, wherein the method furthercomprises the following steps: (f) removing exhaust air from the regionof the building to be cooled, the exhaust air including air heated as aresult of heat exchange between the cooling air and the region in thebuilding to be cooled; and (g) choosing in dependence on thepsychrometric characteristics of the ambient air how much, if any, ofthe exhaust air and how much, if any, of the ambient air are used duringstep (d) to produce the cooling air.
 17. The method according to claim1, wherein step (d) includes a first stage of producing cooling airhaving relative humidity higher than that of the selected single setpoint for the cooling air and having temperature lower that of theselected single set point for the cooling air, and a second stage ofraising the temperature and lowering the relative humidity of thecooling air so produced, by passing it through a fan, so that thetemperature and relative humidity are substantially equal to that of theselected single set point for the cooling air.
 18. A computer programcomprising computer program code means adapted to perform the method ofclaim 1 when said program is run on a programmable controller.
 19. Acontrol system structured to carry out a method of cooling a data centrewith cooling air in conjunction with a source of cooling air for coolingIT equipment in the data centre, wherein the method comprises (a)defining criteria for the temperature and humidity of the cooling air,wherein the criteria correspond to a region on a notional psychrometricchart, said region spanning a range of temperatures on the notionalpsychrometric chart and spanning a range of humidities on the notionalpsychrometric chart; (b) determining the psychrometric characteristicsof ambient air from outside the data centre, the characteristicscorresponding to a single point on the notional psychrometric chart; (c)selecting a single set point for the psychrometric characteristics ofthe cooling air from multiple possible set points spanning a range oftemperatures on the notional psychrometric chart, the multiple possibleset points being in said region on the psychrometric chart, the selectedsingle set point being chosen in dependence on the temperature and onthe humidity of the ambient air; (d) producing cooling air havingpsychrometric characteristics substantially corresponding to the singleset point set so selected; and (e) delivering the cooling air to aregion in the data centre to be cooled wherein the control systemincludes an input component structured to receive information about thetemperature and humidity of ambient air outside the data centre; amemory component structured to store said defined criteria for thehumidity and temperature of the cooling air, and a processor componentstructured to: (i) determine said selected single set point in view ofthe stored defined criteria, and the information received by said inputabout the temperature and humidity of the ambient air outside the datacentre; (ii) determine how much, if any, of air exhausted from the datacentre and how much, if any, of the ambient air outside the data centreare used by the source of cooling air to produce cooling air havingpsychrometric characteristics substantially corresponding to the singleset point set so selected, in view of the information received by saidinput about the temperature and humidity of the ambient air outside thedata centre; and (iii) determine how much, if any, mechanical coolingand how much, if any, adiabatic cooling is used by the source of coolingair to produce cooling air having psychrometric characteristicssubstantially corresponding to the single set point set so selected,wherein the processor performs operations (i), (ii) and (iii) in view ofthe information received about the temperature and humidity of theambient air.
 20. Software adapted to perform steps (i), (ii) and (iii)as set forth in claim 19 when said software is installed onto the memoryof the control system.